Building computers isn't rocket science, problem solving is where experties are needed.
You could build yourself one, but if you ask someone else to build it, at least £100 of that will probably go on Labour to build it for you, leaving you with a £650 computer.
There are many websites that show you how to build a computer and if you have a spare 3-4 hours you can build it yourself.
You could pay someone like Overclockers to build you a machine, but for £750 you aren't going to get much, especially if you also need a monitor.