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Probably because one of the reason people from the EU came to London and, in general, the UK, was because you didn't need a visa.

Remove that possibility, then people will consider other places as well.

I highly doubt people keen on going to London would consider Brussels or Frankfurt an acceptable substitute.

The ones that won't bother to learn a different language certainly won't

Depends on where the jobs move to.

sure but the types of jobs on offer in London may not be on offer elsewhere.