@Clive ********
I just did the maths. Based on the minimum of 65k per month (780,000 per year) required to renew a retirement extension: Thai tax payable on that income would be 70,500 Baht. That's about £1,500 at today's rates. If you paid tax in the UK and it was your only income, you'd have paid £1,060. If you have other income in the UK you could say you paid 20% on the whole amount, which would be £3,455, so your UK tax more than off-sets Thai tax. Of course, if you bring in more than 65k Baht per month your Thai tax bill goes up. But the top rate of tax in Thailand is 35% for more than 5M. The top rate in UK is 45% over £125k (about 5.6M Baht)
So a general rule for UK citizens resident in Thailand is that it could cost us each £440 in Thai income tax. That's £37 a month. Given that Thailand has no Council Tax, we are still quids in. If you think that's too much to live in the Land of Smiles, cry me a river 🤪