corporate-wage-slave

I am presuming that the new UK departure form will need legislation, and until that comes out we can only guess as to how this will work. One thing you currently can do (for most countries) is transit in LHR or LGW en route to your holiday, and I can't see that being forbidden. But if you stay more than a few days in London, and then put down on the departure form that you are off on holiday, I can see circumstances when BA would have to deny travel. It's possible the Statutory Instrument will explicitly exclude those permanently resident outside the UK, in which case you would be fine, but we may have to wait a week or so to see the SI.

More broadly though, I would query if this is the right time for travel complexity. In the last 48 hours countries such as UK, Germany, Czech Republic, Belgium, France, Iran, Portugal and doubtless a few others have introduced travel restrictions.. And even when you get to the vacation location, you may face restrictions on your return flights. So I personally think it woud be wiser to either visit your relatives or to go on vacation, but mixing the two seems to asking for trouble. Indeed I would leave it altogether for a few months if I were you, this really isn't the time for leisure travel.