PBTech are pretty good, and do complete gaming systems. You can also choose your own components and simply pay for them to assemble them, and I think they might have an additional service to install OS and test everything as well. I purchased components and had PBTech assemble my PC, they did a good job, and when it didn't work properly they took it back and diagnosed the problem - a faulty power supply in my case.

I would try playtech or computer lounge.