OJAS

You can actually apply for a new extension of stay up to 30 days before your current permission to stay expires at most offices - and even up to 45 days beforehand at some (e.g. Chaengwattana and Chiang Mai). And retirement extensions are normally processed and issued on the spot at most offices, with no need to leave your passport with them overnight (Jomtien are the notable exception in that regard, though).

I was never required to have my British Embassy income confirmation letters notarised in this way.

Agreed - use it or lose it as the old adage goes!

For the future you might instead wish to consider obtaining consecutive 6-month statements (which Bangkok Bank branches can issue on the spot) at 6-monthly intervals. That's certainly something I did when using the 65,000 THB monthly income method for my retirement extensions based on my Bangkok Bank account, with no adverse reaction from Immigration.

Assuming that your wife is Thai, are you really that skint that you can't even afford a legitimate marriage extension?

It strikes me that, in the case of those of us who have already lived in Thailand for several years, a Thai criminal records check would be more appropriate. But, since immigration offices are part of the Royal Thai Police, one would like to think that they would have ready access to Thai criminal records pertaining to recalcitrant foreign retirees in any event. That said, though, this is, of course, Thailand, lest we forget..........

So how would this work in the case of those who, like myself, haven't lived in their home countries for 15 years?

So presumably all you have needed thusfar to provide in support of your retirement extension applications at Khonkaen have been an account confirmation letter from your bank plus copies of relevant passbook pages? If so, I suspect that what Khonkaen are now requiring for the first time in the case of retirement extension applications based on 400k/800k in the bank are, in fact, 12-month statements, regardless of what the officer told you. In connection with the post from @DrPhibes, 2-month statements would only make sense in the context of marriage extensions (as in his case) which are subject to different financial requirements from retirement extensions. Maybe the officer who spoke to you was labouring under the mistaken belief that you are, in fact, staying in Thailand on the basis of marriage, rather than retirement, extensions?? (Either that or he was talking through his derriere). If Khonkaen are now insisting on 12-month bank statements in support of retirement extension applications based on 400k/800k in the bank for the first time, they seem not to be the first office to have started insisting on this additional requirement in recent times (over and above those offices which have insisted on it all along). For instance, it has been reported on here by @DrJack54 that Chaengwattana (Bangkok) have now introduced this additional requirement. And I personally had it sprung on me as an unpleasant surprise when I applied for my latest retirement extension at Rayong a few weeks ago. Makes me seriously wonder whether some general directive has recently been issued to immigration offices on this requirement by Immigration Bureau bigwigs? When it comes to obtaining 12-month bank statements, it should be noted that Bangkok Bank branches require at least a week's notice to obtain these from Head Office in Bangkok. All other banks should, however, be able to issue these statements on the spot over the counter.

Do you use the 400k/800k bank balance or 65k monthly income method?

Not just a new requirement at CW but also one at my local office (Rayong) as I found out a few weeks ago: https://aseannow.com/topic/1301303-new-rayong-immigration-requirement-for-retirement-extensions-based-on-800k-in-the-bank/ Makes me seriously wonder whether those offices who haven't hitherto insisted on 12-month bank statements in the case of retirement extension applications based on 800k in the bank have now been ordered by Immigration Bureau top brass to fall into line with those who do?

Already done in the case of those whose original visas are of the non-OA and non-OX varieties. But the requirement for criminal records checks would need to be extended to those seeking non-O visas for retirement - which might, of course, require some thinking on Pol Gen Surachate's part outside the "retirement extension" box!

But in the context of retirement extensions would it not be more likely that we would need to apply for a Royal Thai Police certificate (https://www.royalthaipolice.go.th/downloads/clearance.pdf) rather than obtain clearance from our home countries? And where would we be able to submit our applications for these certificates? At our local cop shop? Or would a physical trip to RTP HQ in Bangkok be needed (every year presumably)?

There certainly used to be an alternative to the Government Gateway known as GOV.UK Verify - which did require a UK address to be input as I found out when I tried unsuccessfully to register for this facility from Thailand a couple of years ago. However, as will be evident from the link below, it is no more:- https://www.gov.uk/government/publications/introducing-govuk-verify/introducing-govuk-verify But if, indeed, new Government Gateway accounts can now only be opened with a UK address, this is tantamount to barking madness of the highest order IMHO. What about those who, like myself, no longer have access to a UK address for whatever reason? The UK government certainly does appear to be excelling itself in seemingly being firmly joined-up in at least one particular respect - namely in making life as difficult as is humanly possible for us expat retirees here in LOS (where it is, of course, being eagerly aided and abetted by its Thai counterpart). We already have HMPO and DWP to "thank" for the cumbersomely bureaucratic procedures which have been inflicted on us over the past 10 years or so in the areas of passport renewal and State Pension life certificates respectively. And now HMRC seem intent on joining the "party" by preventing expats from opening new Government Gateway accounts, if your latest unsuccessful attempts are now the new norm.

No dickey-bird mention in the following link of any need for a UK address in setting up a Government Gateway account, unless I'm missing something:- https://www.gov.uk/government/publications/access-our-services-using-government-gateway/use-a-government-gateway-account

Unfortunately, the sad reality these days is that the concept of "joined-up government", as so enthusiatically espoused by the likes of Tony Blair and John Prescott 20 years or so ago, is now completely dead in the water, with individual component parts of The Great UK Government Machine in all probability now acting under strict orders emanating from On High to confine their activities to matters which fall strictly within their purview, with blinkers firmly attached to prevent any "undesirable" sideways glances into areas for which they are not responsible.

Yes, definitely more a case IMHO of yet another shining example of bungling incompetence and ineptitude on HM Government's part - as frequently manifested in our dealings these days with HMRC, DWP and HMPO in particular. To assist you in checking your tax affairs from now on, you might wish to consider setting up a personal tax account, as explained in the following link: https://www.gov.uk/personal-tax-account

Original non-OA visa holders who are unable to obtain mandatory health insurance cover are already required to deposit 3m THB into Thai bank accounts for retirement extension purposes - at least 2m of which is unprotected/uninsured - as a result of sub-para 6.3 of Police Order 654/2564:

Deposits are insured to 1 million THB currently: https://www.dpa.or.th/en/articles/view/protection-limit But in the context of possible requirements for 1.2 or 1.6 million THB deposits your point is still well made IMHO.

In which case why did you singularly fail to make the point (which @BritTim subsequently did) that it was essential for the OP to make his immigration office aware of his travelling plans?

IMHO it is singularly unfortunate (to put it mildly) that you have only now chosen to make that fundamental point clear. EDIT: And yes, I did re-read all your previous posts on this thread before making this comment!

Daily cap for TfL bus travel is now £5.25: https://tfl.gov.uk/campaign/new-fares?intcmp=34454

Plus TSB:

Thaksin to return home on August 10, Paetongtarn says And which cell in the Bangkok Hilton might this be, I wonder?

As is already the case with Australia, I believe. But at least we would then be spared the Life Certificate bureaucratic nonsense which DWP insist on inflicting on us every couple of years or so! ????

But pure guesswork on your part as you say, though. As has already been suggested a number of times, the OP really needs to post on here a photo/scan of his denial of entry stamp stating IMM's specific reason for denying him entry.