A subject very close to my heart this, being as Aero-space Engineering was my subject of choice.
I cannot claim to being in the same situation but I do appreciate where you are coming from.
If you don't mind I'll give a bit of background to let you see my record and judge accordingly.
So back in the late 1960's I had this great plan to get a job as a rocket engineer, not a rocket scientist you understand because rocket science isn't all that much fun and besides I have always prefered designing and building than experimenting and writing.
And thereby hangs the sorry tale.
Back then English was an absolute essential to get into Uni' and I struggled to manage a grade 3 GCE at O level, so I took my A level, Maths, Physics, Technical Engineering Design and Engineering Workshop Practices off to the Royal Air Force where the ability to spell and string words together eloquently were not quite so important, and on the whole I feel it was the right choice for me.
Of course when I came to leave the forces and settle down I found, much like yourself, that my lack of qualifications (despite a not inconsiderbale amount of experience, and dare I say, some small expertise) counted for nothing in the civilain world and I can well understand your desire to bring yourself up to speed. I have to admit that a lack of spare cash and having a young family to support was not conducive with full time study and payment of fees when what was really needed was a steady income, but I have no regrets.
Ironically I managed to land a job that has kept me going for a good many years on the premise that I was cheap and had I been endowed with all those qualifications they could not have afforded me a position.
Lifes full of ironies like that I find.
However if you do not have domestic distractions, are able to do the study and pay the fees then I say do so and work hard at it and be a success.
Your choice of GCSE's is sound enough, although I would consider adding a technical design course of some sort and these are usually offered (or used to be) as part of a study package whereby each subject helps support the others through common themes, but don't take my word for it as I am a fair bit out of date.
One bit of advice that I think is of use, and current, that I would like to give you is that if you can get there then I can think of no better place for studying Aero-space Engineering than Southampton University. It is, in my opinion, second to none in that field.
Hope that's of some help?