I am from UK.originally with full UK passport.

Have been here on "extension of stay based on retirement" for 4 years - Khon Kaen.

Have 800K+ in Thai bank etc.

Never had problems with 90 day report. Next is due 7th August.

No dependents here.

Never had problems when re-entering from trips to neighbouring countries for a few days break.

Always had exit / re-entry visa from KK.

If I go to UK for 7 or 10 days end of this month, that is, ticket Bangkok - London - Bangkok =

Will I need a return flight booked for UK ? That is Bangkok - London.

On my return flight from UK will I need proof of funds here, or here and UK, or some cash ?

Will London end ask for proof of onward flight from Bangkok ?

Yes, if UK leaves the EU rules may change but I can't see that happening overnight.

Thanks for any advise.

You will not need anything other than a re-entry permit. If they ask anything in London just show them your re-entry permit and extension of stay.

If you have a valid re-entry permit a return or onward ticket is not required (it the same as if you had valid visa). No proof of funds will be needed on entry.

Thank you "upnotover" and "ubonjoe" for your very fast response.

This has answered my questions fully.

Now all I have to find is a cheap ticket, can always sleep on the beach when I get there .....

As an aside, I don't think the UK being in or (hopefully) out of the EU will change anything regarding Thai visas.