Do you actually make more, or just on paper? You can't just disregard the difference in living expenses.
The salary for a "Senior Expert" developer in "Everywhere else in UK" is 60K GBP.
You would make more money overall by renting a 1 bedroom flat in London, leaving it completely empty and telling GitLab you live there.
Yes, even accounting for tax.
You could just get a postcode. Ask a friend to take mail for you. Get a flat share and use it as a pied-a-terre. And so on and so forth.
Who knows. They can play the code golf, not me.
(I love GitLab. Their hiring is just odd.).