You'll be hard pressed to find any shops in the UK or Europe that will do custom jerseys to be honest Scottish
Your best bet is to order one from the NFL shop at NFL.com - I got a custom jersey there last year, and they arrive within a week or so, but they arent cheap, especially if you factor in import/customs charges (plus the extortionate Royal mail 'handling' fee - an example is the $95 I spent on the collectors edition of Madden 09, and I had a £16 fee to pay (equivalent of about $30) to collect it - £9 of which was the Royal Mail handling fee!).
If you just want a regular pre-printed jersey, then the usual places over here are worth a look:http://www.ta-sports.net/nfl_shirts.htm www.kitbag.com
(NFL UK use these).
DONT expect a lot of choice though, its usually only a maximum of 2 players to choose from, if that.
Personally, I'd check out Ebay for jerseys - as long as you know what you're looking at (there are fakes on there, but 9 times out of 10 they're blatently obvious), you can get the perfect jersey, for a decent price - you generally have 3 types: The screen printed jersey, the 'sewn' jersy (where all numbers and letters are sewn on), and the fully authentic 'on the field' jersey (they use some weird name like 'replithentic' or something) - the first two types account for about 99% of them online (and anywhere else) and are under $100 for adult sizes, and available in the popular current players, as well as throwbacks (Montana, Rice, Clark, Lott etc). The replithentic ones (much heaavier fabric) are 2 or 3 times more, as are the Mitchell and Ness jerseys, which are exact replicas of those worn on the field in the past (You're looking at $250+ if you can find one).
The other thing to look for is size - adult sizes generally start at Medium, which is a 48inch chest, so for people like me (5'7, 140lbs), I end up buying Youth Xl sizes which is for up to about a 40-42 chest! I dont mind though, saves a lot of money as they are cheaper than adult ones!