It's going to be disheartening with all these comments that you don't want to hear but to be honest they are the truth. The problem these days is that technology moves way to quick and what was top of the range in 6 months won't be. Because of this your build will never be up to date. The biggest problem with the low end PC's is that right now all new kinds of high demand games are coming out, like you've said you want to play battlefield, for a build for the price you want you would be spending more time lagging than playing.
Furthermore i disagree with most previous comments, with a budget of your size if you were going to go ahead with buying one it would probably be better to buy it pre-built, but when you get around £500 that's when you can start buying your own parts. Make sure in any of your builds your GPU (Graphic processing unit) is the part of the highest quality, additionally don't bottleneck it with other **** components but for gaming it's highly important. Also, remember to get a sufficient PSU otherwise you'll find your PC won't last long! Also, for me personally the peripherals i have were equally important to the PC when playing games, between my monitor, mouse, keyboard and headset I've paid £400 so just take into consideration that, playing with a £10 mouse is horrible!
Sorry to keep it so long, but to conclude; I wouldn't buy one just yet, not sure on your age but if you do, can't you wait till Christmas for Christmas money? If you are abit to old for that just waiting a little longer will be so much more worth it in the long run. Lastly, use a website called Overclockers the people on there a superb with this stuff and many are trying to get rid of their 'old' PC's which would be in your price range but out perform all new PC's you would buy. Sorry to keep it long winded, but hope i helped!