“Given that we've already enacted GDPR into British law in the form of the Data Protection Act 2018, they're in for a shock.”
You’re forgetting that once we’re brexited, then a single Act of Parliament can repeal any EU legislation previously enacted using wording as simple as “Act of Parliament xxxxxxxx is now repealed this date of xxx of yyy year zzzz.”
As a sovereign nation any legislation or agreement we’ve entered into with other nations can simply be repealed by our democratically elected parliament.