First thing to do is check out your local bike shops (lbs's) and see what is available. You'll probably want a purpose-built commuter bike, with racks, fenders (a good thing if you ever have to ride in the rain), but what's available varies from place to place. I assume that you are in the UK, since you listed your budget in GBP, so I really have no idea what is available.

Whatever you do get, make sure that it fits you correctly. A good lbs should help you with this and ensure that the bike is properly sized and set up for you.

In my experience, specialist clothing is not necessary at all. I have bike clothes, but the vast majority of the time, I just wear whatever I have on. Its only for rides over 40 miles or offroad rides that I really think of putting on the padded shorts and skintight shirt. Rain gear is nice though.

11 mile round trip is not bad at all- you should be making it with no problem once you get your bike, and might not even want to go back to driving once the year is up.

Best of luck!!