[Symbiote]

Of course the UK could give a unilateral guarantee. Why shouldn't they be able to?

The EU has already offered an agreement to May, but she refused -- it guaranteed more rights (existing rights) than she wanted.

It's also not at all clear who will be able to stay. Students? Pensioners? Unemployed people? Family members? Seasonal workers?

My colleague's daughter will be studying elsewhere this autumn. She was interested in a British university, but her parents didn't want the risk of international fees in the future.