RAF is very very competitive and not taking on any new recruits at the moment. If you want to be an airline pilot they will see it and will not accept you.
The airline industry is a mess, but a small upturn may be on the horizon. There are two main ways to gain your fATPL, which is the license you need to fly an airliner. These are integrated like Oxford Aviation that has been mentioned above, the other route is modular which would be somewhere like Bournemouth flight centre of Stapleford. Many airlines prefer integrated cadets when looking for the first job. Once you have that first job and want to move on your training becomes almost irrelevant. Some airlines like Ryanair don't give a t*ss about where you trained. You must remember that bottom line is that flight schools are a business and they are there to make money, therefore will try and do a hard sell.
an ATPL license requires you to sit 14 ground school exams. GCSE level maths is the highest that will be expected. Trig is crucial, make sure you can write out sides and angles without thought and having to re-arrange. The six times table is also very useful in flying. Maths and Physics A-Levels are recommended but certaintly aren't essential. They may create a good understanding for you when it comes to the ATPL ground school. If you apply to an airline cadet scheme. Maths and Physics A-levels will be expected. However, if you do not enjoy them there it little point in taking them.
My personal opinion is that uni courses combined with pilot training are a waste of time and money. Just get the training. The shiny blue CAA license book will look the same regardless of how you get it.
One thing I know for certain is that it will be very difficult to get your first job. Small turboprop operators like Loganair, Eastern Airways and Flybe are good places. Ryanair also take on new pilots.
It is an expensive business, integrated courses like Oxford and CTC are grossly overpriced for what they give you. Many airlines will also expect you to pay for your Type Rating which enables you to fly a specific type of aircraft. For example, you may be typed on the A320 family for easyJet or 737NG for Ryanair. Once you have that first job you are in a good position to move jobs to more 'career' airlines once you have hours and experience. Many British pilots emigrate to Dubai to persue a career with Emirates. Some will hope to go from easyJet to BA as they operate the same type of aircraft. Some guys will go to Virgin aswell.
The key is that first job, it can be very hard to get. If you cannot immediatly get a job the most important thing is to keep current. An instructors rating is a good idea. An 2 years old CPL/MEIR license is of no use to an airline as you may have forgotten the 'art' of basic flying.
Best of luck to whatever you choose to do.