afaik this is a leftover (as withon the STV application), as covid related insurance is defenitely not needed anymore - have another case running about this and will report back
Maybe it is a mistake and they want to see the 400k/40k there?
STV is very strict on not being eligible to convert into any other purpose of stay visa / extension, surprisingly some IO granted the "unable to travel" extension
If on Visa Exempt, what about getting a regular extension first and sort out the marriage Visa then? The 30 days or 60 days family visit one? Even Covid-Extension?