Windows 8 works fine with new games -- as long as you have a good video card. If the video card is a "bargain" low-end card, you'll be having problems with many new games being sluggish or not playing at all.
There is a problem with some older Windows games on Windows 8. For example, Still Life (2005) and Post Mortem (2002) don't work in Windows 8 -- even the current GOG versions that are DRM-free don't work, and they're two of the very few games GOG offers that aren't listed as compatible with Windows 8. They both work in Windows 7. Games like that are why you want to keep your old XP computer if you get a new computer with Windows 8.
I'd recommend gettin Windows 7 if possible.
You can see from this website that Windows 7 will have extended support until 2020. http://windows.microsoft.com/en-us/windows/products/lifecycle
Extended support means Microsoft will provide Windows updates, including security updates. It's usually all you need. By 2020 you'll be ready for a new computer anyway.
If you go with Windows 8, be aware there is a steeper learning curve than with Windows 7. And you may want to buy something like ***StartIsBack***
to restore a normal Start Menu. Windows 8.1 only gives you the confusing "Start Screen" that looks like a smart phone interface.
Draclvr has put together a good computer for you as an example. See if you can find one with similar specs that's available where you live. You might find one online, or you might find a small computer shop near you that will custom-build a computer for you.
Maybe some of Gameboomers' UK members will have a good idea where to buy.