Not only is it easy to forget your Mathematics over the summer break, A Level is much more algebraic than GCSE so you really need to hone those algebra skills to have a good foundation for September.
Also the link between algebra and graphs is a vital tool which needs to be used with ease when solving A level problems and geometry needs to performed without any difficulty.
Some of the skills you need are included in this new book:
Progress to Advanced Mathematics
by Mary Teresa Fyfe, Andrew Jobbings, Kitty Kilday and Mary Read.
Copies are available in the school library along with other books I have recommended in the past:
A few hours' practice over the summer can really pay dividends next term!