When we sold our house we didn't have fensa certificate for new doors. We considered building control regularisation but solicitor suggested an indemnity policy instead which cost about £12. Not sure if that work for remortgaging too but def worth looking into
We had to do this when we sold our last house. It's about one hundred and something pounds and 52p or something completely not round to have building control come and spend 5 seconds checking that everything is present and correct. You should find the relevant info on your council website.
We had new windows fitted as part of a major refurbishment. We don't have Fensa Certificates for them but believe the Building Regulations and Completion Certificate for the works are enough to cover the "whole job". If not, when we come to sell we'll get an indemnity policy.
I sold a house without one, and I just had to pay for a certificate of indemnity for it. My solicitor arranged it. I think it was about £120, though this was 12 years ago now, so my memory is a bit hazy. It certainly wasn't a huge sum compared to the other costs of buying and selling houses. Annoying, but not difficult.