My information comes from the RAF and Army side of it, but I am 90% sure it is the same for the Navy. To join as ANY officer you need a minimum of 2 A levels grades A-C and 180 UCAS points. In reality, you need at least CCC in A Level, although higher will be better.
If you have a degree, once you pass basic officer training, you will gain extra seniority and pay to account for your university degree. In the RN, without a degree you will start as a Midshipman on an annual salary of £24,615, but with a degree you start as a Sub Lieutenant on £29, 587.
Unless you want to apply for a specific role, such as medical, which requires a specific degree, then any degree will do. Ultimately, I would suggest picking your A Level's and degree based on what you find most interesting, but with half a mind on keeping them as a back up career if you change your mind or cannot join due to unforeseen circumstances.
Hope that helps
Sam