I've applied to the 'big 3', Oxford and Cambridge. I already got unconditional offers from Kings and UCL. I have to accept within 4 weeks and deadline is passing me by.My preference is for other Universities but I'm afraid I may not get a place. Does anyone know if you can accept an offer and back out later if your preferred University offers you a place.

Good question. The same problem bothers me at the moment. I got an offer from Queen Mary which I have to accept soon but I'm still waiting for UCL to decide on my application - which might take up to march according to their letter.

I called the admissions' office of UCL today and they told me that you can always accept an offer and still decline it later if you get another offer. At least that's the case with UCL apparently.