Residential courses in universities are available for pupils in Years 9 to 12. These are held in the school holidays. They are subsidised and typically cost about £145 for a four-day course. Some are more expensive and a few are free!
As far as I can see, the only course with places you can book at this moment is "Physics in Engineering" in October for for young people who will be in Year 8. However, I think they are just about to release details and open applications for 2011-12 so it may be wise to get yourself onto the mailing list if you are keen. The website shows all of the courses which have run or are running this year, which will give you an idea what may be on offer next year.
Feel free to forward this message to other lists. Please remove my details from the message when doing so: I don't know anything about the courses other than what I've read on the website!