What do I need to ensure successful entry into Thailand with a Non-Immigrant O-A Visa before its expiration, and what are my options if I miss that date?

You will need the Thai insurance if you apply for a 1 year extension when you are inside Thailand. You will need the insurance to apply for a new O-A visa in your home country.

If you do not enter before the 2nd of March next year...

You could go through the Non-Immigrant OA visa application process again. As things stand you will need the mandatory Thai health insurance.

Or you could get a Non-Immigrant O visa in London for the purposes of retirement (valid for 90 days) then extend it in Thailand as Robert mentioned above.

Other more complex alternatives are to enter visa exempt or with a tourist visa and then convert to a Non-Immigrant O in country then extend it. However, this may or may not be offered by your local immigration office.

Also there may be a requirement to show that the money came abroad with one or both of these conversion methods.

Using the Non-Immigrant O or visa exempt / tourist visa then convert route, you do not need the Thai health insurance. Or more accurately, you didn't need it BEFORE the pandemic began.

Now, who knows?

There may or may not be pandemic or other health insurance requirements.

Come March, everything might be different again.

If you enter before 2 March 2021 your will get this date stamped as Admitted until date,

You than have to apply before this date for an Extension of Stay, but the requirements are minimum 800.000 Thb 2 months prior application in a Thai bank account and that would be 2 Jan.

If you do not visit Thailand before this date, you have to start with a new visa, there are several options, choose the one which benefit you the best