...you'll be hard-pressed to build for a price that low. The monitor itself takes off about $150 (I'm going to be using the $ sign instead of the pounds sign. The price is still in pounds). I don't think you'll get BF3 on high for that budget. Here's what I would change:
PSU. That's WAY too much power... try maybe a 600W? 550W will be plenty, as well.
Video card. A 6870 is the lowest you will want to go.
Case. That's quite a bit for a case... try a Corsair 400R for a nice chassis. It really depends.
Memory. Get DDR3-1600. It shouldn't exceed $40 (40 pounds, $=pounds symbol)
I think you'll have to cut from the CPU and motherboard. In that case, you would get an i3-2120 (or whatever the number is... Google will autocorrect you). Also, get a lower-priced motherboard. I think you'll be set then.
Yeah , if you have a tight budget u can cut some corners and save some money, the case is really expensive, check out sharkoon t9 value case on amazon.co.uk and a ocz or corsair 500w psu .. thats like 100£ saving, u can put a better GPU or save the cash..