FWIW, Brexit was never, ever, ever about stopping migration.
It was about right wing elites deciding which migrants they wanted, and at best about regaining control of that policy from the EU. It was sold in a racist and bigoted way.
The reason we have less control over illegal migration is exactly Brexit, which, as people were warned at the time, torpedoed our actually cushioned position with respect to "first safe country" asylum rules, and meant we had no cross-border agreements with France.
The "Brexit" that people thought they were getting was always in their own mind because it had no definition. It was an imaginary victory which only has downsides, just as was pointed out.
no it was %90 about that finance and regulation thing for sharing banking and finance data to EU and related, I even fully forgot what was it. It was well hidden and among years it got lost but this was mainn reason. They will not tell you this unless you really deeply read it and see it.