Very good advice - it's difficult for anyone to advise on a bike without knowing what sort of commute you're doing (flat or hilly, main roads or cycle trails?) but go to a shop and tell them exactly what type of riding you want the bike for.

If you're in the Bristol area there's plenty of bike shops locally. I've not used any of the shops in Bristol, but I have used John's Bikes in Bath and Cheddar Cycle Store, and they're both very good shops.