"Great Repeal Bill"
once Blighty departs from the EU's jurisdiction, we will need “a piece of legislation that mirrors GDPR carefully, so as to leverage the fact that GDPR was already put in place...”
No we won't.
That's exactly the intention of the misleadingly-named "Great Repeal Bill", which will transpose EU law into domestic law on exit day. In other words, GDPR (and all other EU law) will stand until the UK Parliament decides it needs to be changed.