Hello mate - saw your question and I thought I would chip in with my thoughts.
You go for it chap.
I wanted to do the same at your age but was academically weak. I'm now in my forties and just embarking on my dream career. I have worked in another industry to get the money together to fund the training and have studied to get the brain sharp enough to do the job.
School subjects; Maths, Physics, IT, Technical, and a language or two.
The first thing to remember is that training costs money so you will need to get that together first. Modular training where you do bits at a time is historically the cheapest and at times of good employment no barrier to getting a job. Integrated training is the other option, this takes you from the pavement to a First Officers job in one process but costs considerably more. Some airlines prefer this method - some don't. If you have access to the sum of money needed to do this say £70-80,000 then it is the most time efficient way to do it.
Now as for the military option it is important to look at this as not just a stepping stone but another career option in itself. Military life is not for everyone but speaking from personal experience it can also be a very rewarding one.
Beware though that becoming a fast jet jock is NOT a given and is harder to get into than a lot of airline jobs. Be prepared to be tested, and pushed and tested and pushed to your limits again and again. Flash job though.
The Military will help develop you as a person and bring out your leadership skills at the same time - all very useful in the cockpit enviroment.
You never know you may actually find yourself liking it and staying. I have a number of friends in the forces who wouldn't want to do anything else - but also some who have served their time and are now looking for a new challenge. Everybody is different. Give yourself a chance to find out what you want.
If you do go down this route - I would advise that you do this with your eyes open and not as a stepping stone. You'll probably go bonkers with frustration waiting to be rotated out if that's all you want.
Right now get yourself up to your local airfield and start looking for any kind of part-time job that is available from emptying the bins to sweeping the hangar floor. Pilots are a decent bunch at heart and I'm sure you will soon be offered a back seat or two which will help you understand what's involved and give you a good background for when you start the PPL.
The important thing is you want to fly. I think that's enough.
