Actually good question, I would like to know how these requests are handled post Brexit. Do A.13 GDPR requests still have to be honoured by the UK up to the Brexit date forever?
Until it is amended or repealed, all European law is brought into UK law through the European Union (Withdrawal) Act 2018. (Formerly, it was brought in through the European Communities Act.)
GDPR is still in force until the end of the transition period (I think Dec 31). What will change after then, who knows? I don't think I've seen anything coherent on any front from the government at this point.
This is the case for many EU laws - depending on the area [0], laws agreed upon in Brussels don't actually mean anything on their own, but member countries are required to write them into their own law. After Brexit happens such laws will still apply, as they have been written into UK law.