Should I apply for a Non-O-A or Non-O-X retirement visa from the UK?

The O-A only requires the financial component for extension. You can set up you bank account etc in the first year and then leave the country just before it expires and come straight back ( I went to KL for a couple of days) pre covid of course. You get another 12 months stamp. You could then start your financial deposit/deposits to prepare for extension in 12 months. If you wanted you could probably leave again and come back in on a tourist visa and then change to O Visa in country.

Can you apply for a non imm from the UK? What's the difference between OA & O?

avoid OX or OA which requires expensive health insurance every year you renew visa, and opt for O visa Insurance requirement for OA going up again very soon from 0 to 3 million inpatient ,*,,

Non imm O 3 month then retirement extension

The question is: "the best route" to what?

Just to be able to stay in Thailand? Or traveling Thailand often? Or how to open a Thai bank account? etc, etc..

"Being retired in Thailand" or original post is too vague, for that only you can find the way based on your priority and preference on many conditions...

and to add, the 90-days Non-Imm-O visa and its extension to a 1-year permit of stay comes without the obligation for a health insurance

. .and there also is the 90-days single entry Non-Imm-O visa for those over 50 or retired. It requires a financial proof and can be extended to a 1-year permit of stay inside Thailand, if one can meet the financial requirements (either 800,000 THB on a bank account in you sole name or proof of 12 months of consecutive transfers of a minimum of 65,000 THB to your Thai bank account). Upon entering you should open up a Thai bank account and transfer the 800,000 THB as soon as possible, because when applying for the 1-year permit of stay, the money needs to be seasoned for 2 months. As you are given 90 days admitted stay when entering, the timeline is doable

You should also consider getting a non-immigrant O visa. I am not sure if you can get it in your home country but you can come as a tourist and get a non-O inside Thailand and then 90 days later apply for a 1 year extension. You will have to open a Thai bank account as soon as you get here to deposit the 800,000 baht. The 1 year extension is the same extension that those of us with an OA get each year. The non-O does not have an insurance requirement.

The O-A will give you time to fully assess whether you REALLY want to retire here, and deal with the "first-time anxiousness" of obtaining a stay extension for every year thereafter. But is always better to get used to the systems here as a younger "OLDIE".

A Non-OA visa is a multiple entry visa that is valid for one year, cost 150 British Pounds, and requires having 800K THB in a foreign bank account in local currency.

A Non-OX visa is a multiple entry visa that's valid for 5 years, cost 300 British pounds, and requires having 3M THB in a foreign bank account in local currency. The Non-OX visa is seldom acquired though, and you should contact the Thai Embassy in London for more information about what the specific requirements are.

Both the Non-OA and Non-OX require medical insurance. Both also require a criminal report from your country's government.

When either visa expires, a person can get a one year extension of stay based on being over 50 at the immigration office. It will require 800K THB in a Thai Bank account seasoned for two months before you apply for the visa, and health insurance.

Which one of two visas are "best"? Honestly, I don't know. You'd have to weigh the cost and the financial requirements.