I was mistakenly kind to the government, as this was a regulation and not a directive it became law in the U.K. automatically. The government did not even have to implement it to become a global leader, they just did nothing and it just happened to them by accident.
But E.U. laws will remain in effect following the transition period unless specifically excluded because it would be impossible to replace every law which came from the E.U. over the past almost 50 years in the space of a few years. The only difference is that they will be frozen as of that date, so if the E.U. make any amendments those will no longer be reflected in British law.
So it is a matter of British law until the government decide to repeal it. But that particular regulation is explicitly required by the Northern Ireland protocol, so has to be applied in Northern Ireland. Which makes a repeal for the rest of the U.K. extremely unlikely. Well, unless the government decide to break international law by violating treaty obligations.
Besides which, the Vehicle Certification Agency has already said they intend to remain aligned with E.U. requirements anyway. And The Road Vehicles (Approval) Regulations 2020, which was made this summer, made it a criminal offence to fail to comply with regulation 540/2014.
And required or not, will it be worthwhile for manufacturers to make different versions of vehicles for Great Britain to those they are producing for the island of Ireland, Cyprus, and Malta? Particularly if it means someone from Great Britain will not be able to drive their car abroad.
The biggest irony of taking back control™ is that circumstances will make it that, one way or another, we will often have the same requirements imposed on us but will no longer get a say (or veto) in deciding them.
It does, under " EU Exit and the Implementation Period" : “Not all EU legislation will form part of UK law after IP completion day. The EU legislation that will be retained in UK law is set out in sections 3 and 20(1), and Schedule 6, to the European Union (Withdrawal) Act 2018”
Section 3 says anything which is currently an E.U. originated law remains in effect.
Section 20(1) just provides interpretations.
Schedule 6 provides exemptions for E.U. legislation which will not remain in law. Which seems to only be those which bind member states to international agreements with third countries made by the E.U., or anything which stem from those agreements. (This is what title V of T.E.U. concerns).