Being a member of cabin crew (whatever you do, don't say "Stewardess":-) is a hard and pressured job. You need to have a good all round education and most companies will want to see a good GCSE in Maths, English and a few more for good measure. What you study at A level is not as important as doing reasonably well at them. It would show a bit of depth of interest if a Travel and Tourism option was available at your school/college. This is the basic side.
What the companies really want is personality, strength of character and a very high level of interpersonal skills - and that is harder to show. You need to get experience in customer service and dealing with people - a lot of CC have started off as check in agents or other jobs that deal directly with people. I have known a lot of ex-nurses, teachers, barmaids, shop assistants etc. working as CC - all those jobs have personal communications aspects. That is where you need to focus - the rest is the hard work. Good luck..