I want to visit the UK next year so that the wife [de facto] can meet my mother. I live in Thailand on a retirement extension.

The wife has been to Australia twice on a tourist visa with the usual guarantees of support, and letter of introduction. Never been problems there, but I was living and working there, for the first visa. I don't suppose I can do the same to the UK as I haven't been resident there for 40 years, and don't hold a UK passport.

Wife has no kids, owns a property, but doesn't work...Housewife.

I've read on TV that UK visas seem harder to get than ones to Oz.

We want to go in 15 months..Any advice on the best way to approach this would be really welcome

No reason why it should be difficult, well over 90% of visa applications are successful.

Your wife needs to satisfy the Entry Clearance Officer that the visit is genuine, affordable and that she is likely to return home at the conclusion of her visit.

The fact that you guys appear to be in a subsisting relationship will go in her favour, especially if you can prove your ties to Thailand.

I'm assuming you now have an Oz passport, though you seem to be entitled to a UK Passport, so whilst you won't need a visa you will still need to convince the Border Officer at the Port of Entry that the trip is genuine and affordable.

Long long way off yet and the rules could change, unlikely but they could, so in the mean time maybe build up a short portfolio of your relationship and ties here, nearer the time collect evidence that the trip is affordable.

Your wife should apply for a General Visitors Visa, but that can only be applied for three months in advance.

See also UK visit visa basics.

I've done both Australia and UK for mine. Not much difference except for the online submission of the UK one. If she's been to Australia and not violated her visa terms, it should be reasonably straight forward.