I have a 30 day visa Exemption (UK) Passport and wish to extend it a further 90 days  Is this possible 

 

 

You can apply for a 30 day extension of your 30 day visa exempt entry

OP, obtain your 30 day extension.

Then prior to it's expiry and prior to January 25 obtain a 60 day covid extension. 30+60 gives you the 90 total your wanting.

Note if in Bangkok you will obtain extensions at Mueang Thong Thani.