Most universities don't require the Computer Science A-Level for their computer science course. So if you can do well on the Maths A-Level, it can show the universities that you are good at that type of thinking.
It may seem pointless to put a lot of effort into revising, but it will determine which university you go to, which will be a big difference in your experience for 3 years. So you could look at the websites of the universities, and if you like a specific university's course you might become more motivated to try to get in.
-Kao (Lancaster Maths & Stats Student Ambassador)