Hi, long time lurker in the forums just signed up to respond to this post.
I'm from the UK too and had considered doing this but when you actually sit down and do the maths you don't actually end up saving a great deal.
This is mainly because we seem to get a much better student discount from Apple in the UK than that which is offered in the US and also because the US prices don't include the sales tax (VAT) in NYC this is currently 8.875% and as far as I am aware is added after you deduct the student discount(if you get it)
for example the discounted UK price for a base 13" retina MBP is £1074 here whereas you'll pay £996 in the states...now that is a reasonable saving of £78 but if you get stopped going through customs when coming back then you could be slapped with 20% VAT which would make your purchase cost over £100 more than if you had bought it here.
yes you could probably get it through customs without paying the VAT but that means binning the box and documentation before travelling.
Personally a potential saving of £78 isn't worth the risk
Also worth bearing in mind is that if you buy in the UK from apple then you get three years of applecare free
to be honest I'd just do what I did and buy your mac the day your student loan goes in, don't wait till christmas for your new toy to try and save £80, those few extra months will more than make up for what you might save