(UK) What to do when deadline to accept an offer is before decision dates of other unis?

[PermanentStudent]

I was offered an unfunded place at Exeter in England in early December and need to accept or reject by February 13th. I applied to Imperial and should hear if I am offered an interview in the coming weeks. I should know if Imperial accepts me by the end of January or beginning of February. I have also applied to Oxford, and have been told that they will be reevaluating my application -- I won't hear about their decision until the beginning of March. I have already emailed Oxford with my situation, and they stated that they will not be able to provide me with an answer before February 13th.

 

When should I email Exeter to ask for an extension on the decision date? Should I wait until the beginning of February or email now?

Is it ethical to accept Exeter and withdraw if I'm accepted into one of my other choices? If I accept Exeter, I have to pay a deposit of about $1500. I have two weeks to withdraw my acceptance and receive a full refund, otherwise I will lose that money. 

This last one may be a silly question... Can universities see if an applicant has accepted a place at a different university? Could a uni not accept me because I've accepted an offer for another uni?

 

Thanks!