Hello Brian, I am Keith, one of the experts on Just Answer, and happy to help you with your question. I am answering this question from Bangkok; I have a Thai wife!
Firstly an administrative point. When you leave the UK send a Form P80 to your tax office. On receipt HMRC will class you as non resident for the tax year following your date of departure and furthermore split the year of leaving into two portions, one resident and one non resident. The form provides for a recovery of over paid tax on departure.
Despite being classed as non resident the rental income is taxable in the UK. Also pensions are normally taxed in the country of origin so from that view point things will go on much as before and you will still have to render annual self assessment tax returns. Any tax paid in the UK will, under the Double Taxation Treaty between the UK and Thailand, be allowed as a tax credit against any Thai tax liability on the same income stream. The Treaty precludes the same income being taxed in both jurisdictions, but it does not protect you from differences in rates of tax.
You will almost certainly have to employ local professional support in Thailand. As far as I am aware if you are liable to Thai Income Tax you only pay on moneys transferred to the Kingdom, but I am not an ex pat; I make only relatively short visits so do not need to be up to speed in this area.
I do hope that my response has been of assistance to you and shown you the way forward.