You will struggle to get a top notch gaming PC for £500. When you consider deducting £70+ for the operating system then you really only have perhaps £425 left. But I will see what I can come up with.
A quick look at a company that has a pretty good reputation found the following.
Nothing between them except one has a Nvidia graphics card the other has an AMD.
I don't think build with your budget is really an option.