External hard drives are much of a muchness to be honest. Consider whether you'll want a portable one though, they will power via the USB port as opposed to needing to be plugged in all the time. That'll cost you around £10 extra.
Apart from that I'd just go to Amazon and see which one is cheapest. Any of the big brands (Seagate, Samsung, WD etc) will do the trick just fine.
EDIT - I have
this one. Had to go through my Amazon history to find out, since I'd forgotten what it looks like. It just sits behind my Xbox One and works flawlessly.