What are the advantages of processing a retirement visa in Thailand versus the UK?

Following

do the 90 day at home and process the retirement visa here to avoid insurance requirements.

Bank account

There is no advantage to doing it in Thailand. It will require 2 additional trips to immigration, possibly spending all day there for the first trip. And it's basically impossible to open the required bank account you need first in order to apply for the visa in Thailand if you don't already have a long-term visa.

You should apply for the 90-day non-O visa while you are in the UK through the e-visa system. Then when you arrive in Thailand you'll already have a long-term visa which should assist you in opening a bank account which you'll need to do as soon as possible. Once you get the bank account open, transfer your required funds into the bank account. As soon as the money has been in the account for 2 full months you can go to immigration and apply for the 1-year extension. That extension should be granted during that trip and you are good for the next year.

If you try to do it coming to Thailand you will struggle to open a bank account to start with. Then once the money is transferred you'll have to go to immigration and apply to convert to non-O. You'll receive an "under consideration" stamp which will require you to return to the office in a few weeks to get the actual visa. Then 2 months after that you'll have to go to immigration again to apply for the 1-year extension.