Basically, go onto the Nottingham website and try and find a contact email for biochemistry or science related applications. Send them an email, outlining in brief, why you want to take a gap year and what you'll be doing (include your ucas number). They should email you back and give you a decision on whether you'll be allowed to defer.
Have you confirmed your offer though? I think the deadline is tomorrow for most unis, if you have, just go ahead and send them an email. If you haven't, then i wouldn't email them now, it might not get through in time. Just confirm the offer then ask them to defer sometime in the next week or so.