Online is going to be your best bet, though I'm afraid I'm not very familiar with UK sources.
There's a lot of variation out there. If you want something pretty authentic-looking, and don't mind paying for it, you'll find plenty of places by googling 'historical reenactment clothing'. If you're on a budget, and don't really care how authentic it looks, then most fancy dress shops will have Tudor/Renaissance costumes.
But, if you're on a budget, and also want it to look good, and you've got a bit of time, it might be worth contacting a local SCA or other historical reenactment group. They might be able to put you in touch with a costumer who could make (or help you make, depending on how keen a sewer/crafter you are) an outfit.
I did a quick search, and these are a few links I found that might be helpful:
http://www.thehcc.co.uk/http://www.histrenact.co.uk/societies/medieval/traders.php