Anything by Sennheiser. They make some great headphones and even on the lowest setting they'll shut the world out. I have the Sennheiser HD280 Pro. 9 feet worth of cord and I do look a little weird when I use it in public, but it's great. Very comfortable, I got them at $100, so less than £50. Had them for about 2 years now. I use it for movies, music, and games. It's plugged into a M-Audio Revolution soundcard.
I purchased the Sennheiser PC-350 headset over the summer and am extremely pleased with them. They cost around $90 on eBay, though a thriftier shopper could probably get a better deal. They're of high quality, are less prone to accidental breakage because of their design, and most importantly are very comfortable for long periods of time. I would highly recommend them if you could find one within your budget.
+1 for Sennheiser. In my experience, they are the best sounding headsets for a reasonable price. I've also heard good things about the SteelSeries, but never used myself. Me, I'm a neat freak, and went with the Plantronics .Audio995 wireless headset. Sound quality is 6/10, Mic quality is 8/10. Battery life is about 10 hours per charge, but they use an internal battery . But, they're 100% wireless, and rather comfortable. I paid 25$ US new on ebay, but they're anywhere from 60-100 new.
Well, if you look at the technical data of each brand, they're close in specs. For example, the SS Siberia has a response of 18-28k HZ, and the Senns have a response of 10-26k HZ. Siberias have a driver of 40mm, Senns 38mm. But, the SS have a MSRP of 65, and the Senns are 229. It all depends on what you'd like to spend. I'd personally use either brand, they're both very solid and will give you good sound quality, although to me, the SS look a little more comfortable. In terms of real hardware, those two are a ways removed from a department store