That budget is really tight but I made it. This doesn't cover the price of an OS (£69.29 for Windows 8.1) and the price of a slim DVD drive (£14.99) though, as for upgradability, you can always add a MSI R7 250 2GB GDDR3 (£62.95) to Crossfire with the iGPU of the A10-7850K for better frame rates in games. I got to warn you though, if you go with the R7 250 route, make sure you buy another case, the one I listed doesn't have an expension slot for the video card. You won't need the R7 right away anyway, you can get playable frame rates easily at 1080p and even more at 720p.
Edit: Prices might drop a little by christmas so you're in luck.
Edit: One of the pros of this build is that it is highly portable, no bigger than any next gen console so you can bring it anywhere you want as long as you got a TV with either a HDMI, VGA or DVI port to play on.