Before I started building my own I had pcs built to my own spec by a couple of British companies, notably DinoPC.
When I priced up the parts I'd chosen at retail prices I found they hadn't charged much for building it and setting it up etc and I got a warranty. Well worth it and there are a number of companies that will do this.
I think it was about £60 for a £900 build so, for the warranty and peace of mind, it's almost not worth building your own.
Mind you I enjoyed building my own and I glow every time I switch it on