This feels like one of those things where Post-Brexit-Britain simply HAS to do something different. Just for the sake of it. Sovereignty and all that. The fact that many online businesses will probably still cater to EU citizens and will therefore still follow GDPR rules doesn't matter. Those companies will end up with two slightly different sets of rules and it will just be annoying.
If you're an online business established outside of the EU with no presence in the EU then as long as you abide by the data protection laws of your own country I don't see how an EU country's data protection authority could do anything to you.
This is a general issue online where sites and potentially services can be reached and used globally whilst each country basically cannot do anything outside of its own borders.
The part of the GDPR that says that the regs apply worldwide as long as the individual is in the EU is not really realistic in many actual situations.
