There are lots of plumbing merchants spread around the UK. A quick search on the Internet or a flick through your local Yellow Pages Directory will bring up a list.
You can normally order goods online, by phone or by calling in to your supplier. Most will expect you to open an account. Goods can be picked up from the trade counter or be delivered direct to you. If you order by a certain time, and a certain amount you should be eligible for next day delivery at no extra cost.
Plumbing merchants are a good source of information, they are usually happy to assist in the search for products or fittings.