I'm afraid I'm with the others on this. If you decide you need to change to 1/2" from 3/8" and you need to buy 12 barbs - OcUK aren't going to help you. They'll sell you a complete kit, or some high-value parts, but they don't carry the spares and accessories required to support a water-cooling user.
Now, on that basis - I would buy all my parts from a water-cooling specialist as I want that person around when I need assistance.
At the very least you want someone who will make an effort on your behalf, and I don't see that at some major (r)etailers anymore.