Just because you have harmonised regulations, doesn’t mean you have harmonised interpretation and application of those regulations. The UK have always had much tighter regulation of its financial sector than other EU countries, always being a step ahead of whatever process the EU introduced. France and Germany are also good but the other EU states are a relative joke.
You succeed as a financial sector by being more stable, not more volatile. Since Brexit, the PRA and FCA have been trying to figure out what from the EU regulations works and which of it needs more tightening, more than which of it can now be dumped.
There’s also plenty of precedent for the EU accepting “equivalence” from other territories regarding financial regulations. This is saying that they recognise there are differences but the controls are equivalently strong, meaning it’s ok to have access to the EU market. Switzerland would be an example — FINMA don’t apply Solvency II but EIOPA grant their regime equivalence.