Study/gaming laptop, £800

I'm looking into laptops for university in September, with an absolute maximum budget of £800. Cost efficiency is my main priority though, after power.

I'm mainly looking for a cheap CPU/GPU pairing that could run Skyrim, Deus Ex etc. with at least a good draw distance, and I can live with 30 FPS. 15" screen would be fine, and I'd be more than happy with replacing any parts myself with better, cheaper (cheaper than upgrading from in store, eg. Dell) components.

I'd prefer to avoid HP, going off what I've heard about their unreliability.