Actually universities discourage this. From Cam website
Qualifications taken early
'We are, of course, in favour of stretching and challenging learners, but not at the expense of levels of achievement. Thus, we would discourage schools and colleges from entering their students early for public examinations unless they are very confident that top grades will be obtained. A grade B in a AS Level taken in Year 11 is still a B in our eyes; it is not equivalent to an A in the same qualification taken in Year 12. Where students are successfully taking qualifications early, we would still want to see evidence that they can cope with a workload equivalent to three A Levels taken simultaneously.
We must also highlight the potential disadvantages of taking A Levels early in subjects where the knowledge and understanding will be required at university. Students who have not studied a key subject in a structured way the year before they arrive at university can find that their knowledge, all-important to some undergraduate courses, has atrophied. A good example of this would be a student studying a maths-based course like Mathematics or Engineering whose maths has become rusty during a pre-university year in which maths has not been systematically done.
If a school is looking for ways to stretch its most able students in Year 11, we would strongly recommend that they encourage students to read widely around the subjects that interest them. This might be facilitated by the provision of appropriate reading lists or even material; and appropriate follow-up, for instance via a discussion group, should help develop the sort of analytical thinking and intellectual flexibility that we look for in applicants - and which is central to success at university.
Another way in which they might consider stretching their most able students in Year 11 is via Critical Thinking AS Level. This may develop students' thinking in a way GCSEs do not, and, as we do not include Critical Thinking in conditional offers, there is no risk associated with taking this qualification early: a lower grade will not damage an applicant's profile.'