Most of the guys I work with ride hybrids to work, for their durability. There is a huge amount of hybrids in the £500 to £700 range. They will have Shimano Altus, Acera or Alivio group-sets and disc brakes. Some may come with carbon forks as well.
The Cannondale Bad Boy might be a good fit, if you are a bigger guy. Its a hybrid with a slight lean towards MTBs.