Yes this is the requirements for to apply for the regular 90 days Non O visa as a e-visa in the e-visa application online through the embassy in London.
You don’t need any insurance to apply for a regular 90 days Non O visa at the thai embassy in London. (Only Non O-A visa). And you don’t need to prove accommodation. If you apply for a regular Non O there’s no mandatory insurance requirement for the one year extension of stay in Thailand. (As I guess you mean with the one year retirement visa).
You don’t need to wait until your 50 years, but you’ll need to prove £ 9,300 to apply and two months each year prior to your yearly application for extension of stay.
Some immigrations accept proof of a monthly income of minimum £ 930 for your yearly extension of stay.
That’s the requirement for apply for a Non O visa based on marriage and yearly extension of stay in Thailand.
If you apply for the Non O visa at the embassy before you leave, this is the requirement at the Thai embassy in London.
Nothing against agents who earn a living and do a fair job for a reasonable price, but there’s always a bounce of agents willing to “help” you for 20-30 000 baht by doing a job with real cost 1900 baht and you’ll need to provide most documentation yourself, or agents who are even willing to lend you some money for some days so your whole stay technically is illegal. And you see here, OP’s agent didn’t even advise him fairly be telling him the other option with the income method because he could lose a client.