There are others that I’ve seen mentioned and it’ll be upto you to do the research and make the final choice. All I can say is that my experience was absolutely fine.
I think you’ll have to mail your passport to the Embassy in London just like I did. Other members of this forum may be able to comment if they were in similar position as yourself. The CoE is emailed. I obtained health cover online from Tune, a Thai insurer. Cost is B8k for 90 days and B20k for 180. Good luck with your application.