I don't want to buy my parts in seperate locations
Usually the cheapest way though, its a hassle but not all places have the best prices. You can at least use that website to see where you can get the biggest savings then buy everything from there.
Although I'm not in the UK, I can tell you usually online is the cheapest place to buy parts, never buy from small local shops unless you're desperate because I've never seen a small shop not rip off its customers. Big Box stores (whatever you have in the UK) usually sell for slightly under the MSRP and sometimes will have sales.