I want to go back to the UK for a few months I have a new Non 'O' "retirement"  visa. I'd get vaccinated whilst there. As under the present system I'd need to have the equivalent of 500.000 baht in the bank for 6 months would my 800.000 baht in a Thai bank qualify? 

The 500k baht in th the bank is not required if you have a valid non-o visa or a extension of stay based upon retirement with a valid re-entry permit.

Requirement are here. https://london.thaiembassy.org/en/publicservice/119247-requirements-for-certificate-of-entry-during-travel-restriction?page=5d6636cd15e39c3bd00072dd&menu=5f4b6eb3f6ae4b236972c562