Nobody is talking about an OA. There is no A-O. The visa you are looking at from London is a single entry non-immigrant 0, based on retirement. Extension of 12 months is available in Thailand.
There is no contract commitment only need to buy a monthly credit as and when needed. So if visiting for a few weeks a £10 1 month credit will give unlimited UK calls and texts and 12gb or so data.
Indeed. If it's purpose is mainly/only to receive texts from a UK bank from time to time then all that is needed to keep it activated is to send a text every 6 months. A £10 credit will last something like 15 years. Ought to be in line with the budget of even the most frugal "expat".