@Mandy ***
pardon me I was just trying to save you a very costly trip to the UK... And the quarantine requirements of both on arrival plus in the UK and return to Thailand.
Is it possible to cancel your existing trailing visa /extension...
Apply for the COVID extension...then apply for the Non b for teaching?
As for the COVID insurance... Ask the Thai Embassy in UK.
If you do obtain a new non b in the UK... You will be stamped into Thailand upon arrival for 90 days.
That is the only COVID insurance proof that you will need to submit to the Thai Embassy in UK for your CoE.
And to get that Non b for teaching in the UK... You will need to have this....
As a teacher do yourself a favor...
Get a teachers contract..
Get the WP3 Thai work permit in cooperation with your school employer...
Have your school contact the Thai Ed Ministry to get your name placed on a list of teachers managed by the Thai Foreign Ministry...
Make application for the Non B visa for teaching ...
The Thai Embassy will look at the approved list and issue you a Non B for teaching and a CoE to enter Thailand.
Without these steps followed you will not get the Non b in the UK.
Good Luck