Most migrants believe it or not don’t try to get to the UK.
The ones that do would have various reasons: family, the fact that the UK is “the end of the line” with a sea border that’s tricky to cross but perhaps harder to defend, the likelihood perhaps that they’ve been told by some traffickers that post-EU it would be harder for the UK to “return” them (Priti Patel, ridiculously, claimed the opposite).
The UK’s “benefit” system that’s much talked of is actually nothing to risk your life in a boat for. However employment prospects would be greater than in France for example , which has identity cards and where it’s very difficult for unregistered people to get jobs.
Much easier here.