First, you don't save any money building yourself. A few years ago you may have done but now Dell and others can buy the parts so cheaply they can built it cheaper than you can. Plus they get Windows cheaper than we do (as they buy in bulk).
You may have big trouble building one for £200 but may get one for £300.
I have built a number of PCs and usually find the hardest thing is to sort out is the right combination of motherboard, CPU and memory.
The two main CPU makers are Intel and AMD, and motherboards are made to take Intel or AMD CPUs. You cant put an Intel CPU in an AMD motherboard and vice versa.
I think the best thing to do is buy a motherboard bundle when the motherboard, CPU and memory are already put together and tested. This means you know they all work together.
Novatech are one company who sell motherboard bundles
http://www.novatech.co.uk/motherboardbundles/
It is satisfying building your own PC, but the first one can be a bit daunting as you open the case and wonder where all the cables go.