So much on this, and understanably so. I myself am likely to come into the situation of a frozen state pension in a couple of years though I do have a workplace final salary pension as well so the basic state pension is an "add on".
Like many I have wondered how the Philippines qualifies for increases but not Thailand. This is due to the bilateral agreement entered into between the two countries. This was signed in 1985 and came into force in 1989.
I also wondered why Thaialand has not been approached to do the same and like some presumed this was likely because Thaialand did not or would not agree but after a little searching I came to unedsrand that the Thatcher government, elected in 1979 announced in 1981, as a part of policy focused on reducing public spending and tightening government commitments, especially regarding overseas obligations like automatic pension increases for expatriates stopped considering or pursuing any further agreements.
The Philippines SSA was already in discussion and was allowed to continue to signature and passage into effect but no new agreements were considered.
So it seem as if this is another Thatcher, or more specifically Geoffrey Howe's doing.
I think it is an important part of retirement planning to understand what the implications of retiring in another country are, and to research all financial aspects especially. Anything less is like expecting your host country to honour your UK library card or take payments in pound notes.
So far as paying tax on pension payments is concerned, if the money was earned (arising) in UK and you were in PAYE then I think your National Insurance payments were free of tax, this part of your income was not taxed at the time you paid into the scheme. Clearly as income invested for later payment (pension) it will be taxed somewhere. The alternative would have been a higher tax bill whilst in work. I'm not 100% on this bit but is how it was explained to me.
Bob