OK, the below will be the last bit of advice i give you on the matter as i don't want to stress you into doing something you're not happy doing (whatever you decide is right for you - is right. It's not worth stressing over).
That said, there is a definite advantage in having the CPU, motherboard and memory coming from the same supplier (ideally a dedicated computer/component wholesaler). Sole reason is that if you run into problems during the build it's sometimes difficult to isolate the component at fault and it's of huge benefit being able to send back the CPU/MB/memory back to OcUK who will have the facilities to isolate the offending part for free (that is if the problem was ambiguous enough to consider one of these 3 parts as the culprit - which isn't that unusual (well, would be unusual for CPU)).
My point is that building a whole new PC from scratch, without having spare components, can sometimes have its issues - especially if you encounter an ambiguous error. Being able to return 3 of the core components for testing to a computer specialist, for free, to rule them out quickly is usually worth paying an added premium on parts or sometimes a days wait...