They use the term 'retired' if you are over 50. You don't need to be retired to get one, just be over 50 years old and meet the financial requirements, 800kTHB in a Thai bank. The application cost for the non-o visa is 2000THB and each annual extension after that is 1900THB.
How does that compare against the costs of an OX visa?
then do the leg work in the UK, get the TR60 and keep it in your pocket, so to speak. It will save you time applying for one in a neighbouring country to Thailand, which can take a few days before you return to Thailand. Also note that you are allowed only two visa exempt land crossings into Thailand per calendar year, tourist visas crossing land borders don't get counted in that, neither does flying in. Extend each entry by 30 days. Find a more suitable long term visa that you can meet the requirements for, if needed.
You can apply for the non-o retirement in Thailand from a tourist visa entry or visa exempt entry. It needs renewing annually, no need to leave the country. No mandatory insurance cover. 1900THB per year after the initial 90 day non-o 2000THB.
Opening a bank account is the biggest challenge. It's easier if you arrive on a non-o visa or non-oa however it's not impossible on a tourist visa or visa exempt. It's all down to the branch manager of that branch. Many people employ an agent to complete this step.
Note, if you come with the non-oa 90 day, you can't change that in Thailand to a non-o and you'll need to pay the mandatory insurance for every further extension.
Another option to consider is using a visa agent for the whole thing, although choose your agent wisely. Be aware if you start down that track, you'll probably be stuck with that agent for each extension, 20-30kTHB per year but no money needed in the bank.