If you own a second home, you might want to consider making an election under TCGA92/222 (5). Such an election can be changed at any time, but if this election is not made, HMRC will judge your primary residence based on the facts as they judge them.
By making such an election you can potentially save Capital Gains Tax (as it is normal for your primary residence to be CGT exempt). A second, less well known benefit is that any property that has ever been a PPR has a three year period added on, where the gain is free of CGT. So in other words, if you lived in the flat shown in the picture for 1 week out of an ownership period of five years. CGT would not be payable on 60% of the gain!
As to working out what the situation should be on sale, this is beyond the scope of this article; but the key point is if not done you cannot subsequently alter your election.
More information ‘from the horse’s mouth’: