Those are slightly strange A-Level choices for someone who wants to be an aircraft engineer or pilot. Not wrong choices just unusual.
Your idea is an OK idea but in practice I doubt it will go to plan. You will be 35 or there abouts when you plan to leave the military. No wife, no kids? Maybe not in your plan but those things usually become part of life and good luck saving up GB£150,000 on 17-30K a year.
Now here is what I would do unless of course you would love to be an aircraft engineer in which case joining the military would be a good start or working towards your civilian licenses. I would go to university, join the university air squadron, get a part time job and have the occasional flying lesson with the aim of having a PPL upon graduation in 3 years. If you really would like to be in the military 4-5 years is a long time, maybe the RAF will be looking for pilots but more importantly you will be far more competitive for direct flying positions in the Army Air Corps or RN with a degree and more life experience.
You are at an age when you can make this happen, at 35 with not enough money saved up it starts to work against you, family, kids, responsibilities all make it harder. If you have kids can you spend all your savings and probably take out a descent loan for an unlikely chance at 35 of being an airline pilot? At 17 you may think so but with a few more years under your belt you will most likely not agree with yourself today.
Here is another idea. The chances of finding employment in Australia are much better than the EU due to more GA. If you go to uni in Australia you can end up with the right of abode, alternatively study for a qualification that is on the immigration list for immigrants to Aus so you can go live there once qualified. You can make a lot of money there working in the places no one else wants to. You could save up the funds for your license much faster and after the qualifying period would be able to work in any field not just the one you used to get there in the first place. Get a license and get a job flying. Your chances would be far greater with the right to work in EU or Australia.
Not saying that's the best idea just another to consider.