If you're planning to continue to an earth sciences degree then just doing a breadth of basic sciences and maths is probably the most suitable preparation. I gather of the basic sciences chemistry is particularly useful.
Do bear in mind ad with all science degrees, earth sciences and geology are necessarily mathematical at degree level and you really do need to be able to cope with the A-level Maths material - if not required for admission you will certainly cover this in the first year of the degree. However it is much better to do this as an A-level where you have far more contact hours and individual support from a teacher rather than limited contact hours on a lecture hall of 200 students where you will have limited opportunity to have detailed individual support.
Chemistry, maths, and something else (physics, biology, geology, geography, etc would all be relevant in various ways) might be a particularly useful background to prepare for that.