unless you intend to do computing at uni...I'd say don't spend more than 700, and wait for all the "back to school" offers that are around september next year....
especially as intel are about to release the sandy bridge processors within the next month or so, so you might as well wait a bit
If it doesn't interfere with your course too much, get a MacBook Pro - they are fantastic!
MacBooks are pathetic, you'll be learning C or VB.net (most likely, anyway, afaik), you don't need anything special.
Wait until you're looking at university before investing in a high priced laptop, from what I'm told all software-related work in A Level ICT gets done during classtime and you'll only need your laptop for typing up coursework. By the time you decide what to do after A Levels your needs may have changed to the point where a specific computer/brand is essential, and you'll get better value for money then too. Speak to whoever will be teaching the course and see what they recommend.
That being said, Apple machines can run both operating systems (Even simultaneously using a program called Parallels), which means you wouldn't have any operating-system based worries. Getting one would give you access to any Windows-exclusive programs while a Windows machine has no way of running Mac-exclusive software (Most notably Final Cut Pro, the industry standard movie editor. Not something that you need to worry about for your A Levels, but if you planned on pursuing this branch of media further then you'd essentially have to get a Mac)
so would I also be able to play heavy games on a macbook pro?
so is there a mac version of microsoft visual studio?