You should replace your tyres in pairs, as the two fronts should wear at the same rate (and the two back ones at the same rate) - so if its only one tyre thats below the legal minimum, the one on the other side of the car is likely to be very close to the minimum anyway. This also eliminates any risk of dodgy steering due to different wear rates/compounds etc, etc.
I had hankooks supplied as original equipment on my new car, so they are a decent brand thats used by car companies even if they are not seen as one of the "prestige" brands.
If your a tesco clubcard user, try tesco-tyres.com (its a rebranding of the blackcircles service). The tesco site had Kumhos on offer, and they'd probably be fine if your a normal driver.
You may want to consider the lower rolling resistance tyres (e.g the Hankook Kinergy Eco) as long as they are only a couple of quid more expensive, as you'll recoup that in reduced petrol costs.