It’s completely fine to call me Jane! Only a few hundred million Jane’s in the world…
I have only ever sold a second home in the UK, so the position was very simple. And v little capital gain anyway! But it does seem there is a rather large grey area about main residences that you aren’t occupying, but are yet to acquire another main residence. So rather hard to be definitive especially as the final judgement will be with the tax inspectors tho’, and they can be stubborn.
Declaring UK rental is easy peasy! On both sides of the Channel.