How do deposits work when I'm choosing between American and British schools?

In England they give you a conditional offer, so I’m not actually accepted to the school until I get my AP scores back. The deadline to commit is May 3, but I’m allowed to select a “firm” choice that I will go to if I complete my offer and an “insurance” choice that I may go to if I fail. I got accepted to Imperial, UCL, Kings, and UC San Diego. Can I put down a deposit for UCSD and also firm Imperial so that if I don’t get in I can go to UCSD?

I would

What happens if I tell UCSD I’m going to go and then don’t go?

You lose your deposit. That’s it. Many people change their mind or circumstances change over the summer. Students get off wait lists, etc. Not a big deal.

Just be certain to let UCSD know that you won’t be attending, so that you aren’t billed for classes, housing, etc.