If you're paying for tuition and accommodation, you won't be able to spend much on your own, depending on where and how you're going to spend it. It's certainly not enough if you're going to be frequenting bars or designer stores, but £200 a month is more than enough if you're just going out for the occasional meal. I didn't even spend £300 a year when I was in high school.
Your personal expenses are generally in three areas, i.e. cell phones, transportation and food. Cell phones average around £30 to £40 depending on how much you can call, and if you don't call much you can save a lot of money by signing a contract that gives you 600 free minutes for £20 a month. If you can't get 600 minutes, it's still a good deal.
Transportation includes the cost of getting around, such as buses, cabs, trains and the tube. If you're living on campus these may not be much, but the occasional weekend away for £2 or £30 should be enough, provided you're not crazy enough to take a taxi.
If you don't like the school food you might want to go out to eat once in a while, and sometimes you'll have to treat your friends and so on, which is a bit costly, and you might not be able to beat 100 pounds if you eat often. If you go out to eat once every 1 or 2 weeks, £4 or £50 should be enough, depending on what restaurant you go to.
The rest of the cost is for some things, such as the occasional dress, a gift for someone else, stationery and so on. These can cost anywhere from 20 to 50 pounds. But it depends on where you are.
Just don't learn to be too extravagant. I know some idiot who can't spend £4000 a week and goes to a hotel almost every day, you're not going to be like him are you?