The brand doesn't tell you much.
Get one with Reynolds frame if you can afford it. It will be lighter and stronger.
Alloy wheels are lighter but are more easily deformed, so there is a trade off.
Centre pull brakes are better than side pull.
GET A DECENT LOCK !!!!!!!!!!!!!!!
If you don't have a decent lock your bike will vanish within a month. D Locks (horseshoe locks) are the only type worth having, chains are useless. It will set you back twenty quid, but it is worth it. They can be jacked open given time, but theives look for chains which they can cut throuch in half a second.
Your nearest cycle shop (not Halfords) is probably the best place to buy. Cycle shops are usually run by enthusiasts. They are not out to get rich, so don't be afraid to ask for advice from the staff.