What is the correct order and requirements for retiring to Thailand from the UK?

The people here mean it all very well. But there are different opinions and many are wrong. I follow immigration closely because I find it interesting. What I really advise you, use e-visa directly through the embassy or have it arranged by a visa agency. Then you are always sure of your case.

Non-O you don't need health check or insurance. Apply for it online at Thai embassy. Valid 90 days to use it to enter. Gives you a 90 day stay so you can open bank account then apply for 12 month extension of stay based on retirement..

I'm assuming you are talking about the non-O based on retirement (being over 50) and not the marriage visa.

You apply for the 90-day non-O visa from the Thai embassy in your home country, using funds in your home country as proof.

When you arrive in Thailand, you go to your immigration office to apply for a certificate of residence. While at immigration, ask them for their handout showing requirements for the 1-year extension.

With this certificate of residence and your non-O visa, you should be able to open a Thai bank account. You will likely also need a Thai phone number for this.

As soon as you get the bank account, you transfer 800,000 Thai baht into the account, as you need to show this money in your account for 2 months when you apply for your 1-year extension. This means you only have about 1 month to get the account open and the money transferred.

Gather the rest of the documents from the list you received from immigration so you can apply for your extension once your funds have been properly seasoned.

This method has no requirement for insurance, no health check, no need to translate any documents.