Can you obtain a Non-Immigrant O multi-entry visa at the Thai embassy in London?

I can't answer your question but is it really desirable to have a multi-entry visa versus a single entry in this time of quarantine? Wouldn't you have to re-enter Thailand every 90 days and go through whatever quarantine is in effect at that time to gain another 90-day permission to stay?

If you get a non-O visa you could extend that for one year permission to stay in Thailand if you meet the requirements to do so. In that case, you would not have to leave and re-enter Thailand and face whatever quarantine remains in effect at that time.