How would customs know? Do they have the power (or maybe more importantly the will or resources) to have you prove it was a UK purchase? How would they detect this?
It's just that one can definitely take a trip to the US and have a holiday into the bargain...just saying
I thought I should reply to your comment before you have customs charge you a big duty & VAT bill!
I'm sure that there's lots of people who have got away with it and some will suggest dumping all packaging to make it less suspect (and you'd best also dump the paperwork from the US authorities claiming your purchase tax back upon leaving the country!). If you are walking through the green channel at the airport and UK customs do decide to stop you, then it is UP TO YOU TO PROVE TO THEM that the camera was bought in the EU. If you cannot do this they can charge you the duty and VAT and it will be up to you to seek a refund by providing EU receipts for your gear. On top of this, they can also fine you for evading UK taxes.
It's up to you whether you wish to take this risk to get a camera for a couple of hundred quid less. Many people advise taking copies of receipts and inventories with you when you travel to avoid accusations of importation.