Can I apply for a 60-day Thai visa in the UK and then a retirement visa in Thailand?

The non O does not have an insurance requirement, you are probably looking at the OA. You are also much better off applying for the non O in your home country before traveling

800k baht or equivalent here or there

If they are asking to much money in the UK , then do you think they will ask for less.in Thailand,or am I missing something.

If they are asking for insurance you are likely looking at the wrong visa option. Insurance is required for the 12 month Non-OA visa, you should be applying for the 90 day Non-O visa which has no insurance requirement and can be extended inside Thailand every 12 months, you still need to show you have sufficient funds though and you'll need the funds to obtain the extension anyway.

If you don't already have a Thai bank account you need to get the visa before travelling as you can no longer open a bank account on a tourist visa.