Hi,
I used to build my own pcs 286 - early pentium and saved hundreds of pounds but I now buy complete systems because:
a) they tend to be cheaper than I can build them for.
b) when something goes wrong the whole computer is someone else's problem and they can't say 'not my problem as that bit you got from someone else is what's wrong' and the other supplier then disagrees and you are stuck in the middle with a system full of guarantees that doesn't work.
(Incidently I can also vouch for Gertrude's suggestion of a local Cambridge supplier).
When buying Macs there is no real choice as I have to get complete ready built base units.
If you do build it yourself my top tip is to make sure that you have at least one elastoplast beside you as I always end up cutting my fingers on the edges of the cases - even modern ones.
I hope that the following isn't too commercial for PPRuNe but I have recently purchased a Dell laptop at less than half price from Dell's own e-bay shop. Here they sell 'scratched' units in e-bay auctions and after checking out the specs of the computer and the type of scratch/damage involved I entered the auction and won a £1700 laptop (based upon building a similar computer on the main Dell website) for under £800 (6 other bidders). When it arrived it appeared to be brand new and had all the software backup disks etc. There was a minute scratch that I wouldn't have known was there unless I looked for it and it also came with a three year Dell warranty. I mentioned this to someone else at work and they have just won a scratched desktop (no monitor) and are very pleased with both it and their savings. (Though the scratch appears to be a little bit bigger on their desktop).