I have £800 and need a prebuilt gaming PC.

And mate.don't buy prebuilt pc
Just go to computer shop and give ur requirements for them and they.will assemble for you free!
You can select what you need
Don't give away 800£ as someone else wants
 
Why you want prebuilt one?
Those are overpriced
VISIT computer shop
Give about parts list
They will assemble your pc within hour with better warranty periods and for much cheaper!
That's what you should do!
You don't have to fix parts
Just give your list to them!
Good luck