Can a UK citizen apply for a long-term retirement visa in Thailand after entering on a 30-day visa exempt card or SETV?

I'd say given your situation that you might as well get a single entry tourist visa and then come into thailand.

That would stamp you in for 60 days (and you could extend it for 30 more if you needed to).

That would buy you enough time to open a bank account, get the funds transferred in and let the money start to season so you could apply for the 90 day Non-O and then the yearly extension.

You certainly could do it on a 30 day visa exempt entry BUT a tourist visa would buy you more time to get everything sorted out

You certainly could come in on a visa exempt entry or a single entry tourist visa and then in THREE trips to the immigration office manage to get a year long extension of stay based on being over 50 (retirement).

The way it works is you'd come in, go to the immigration office and apply for a 90 day Non-Immigrant Type O visa. Your application would go under consideration for 14 days, then you'd go back and get the Non-O visa and a new 90 day stamp inked into your passport. Then you wait until you have 30 days or less left on that stamp and go back to the immigration office and apply for a yearly extension of stay.

You would need to meet the financial requirements for both the 90 day Non-O visa and the yearly extension of stay, by either banking 800K baht in a thai bank account in your name only OR by getting an affidavit of income from abroad from the UK consulate here stating you get over 65 K baht a month in income.

Why not just apply for a year-long, multi-entry Non-Immigrant Type O-A (Long Stay) visa from the thai consulate in London BEFORE you wing your way here?