My experience for what its worth...
I worked whilst I was training.
I started my PPL before I had a medical then got the medical before my first solo flight for 2 reasons A) Cost (saving for it) B) If after a few hours in the air I realised it wasn't for me the losses were minimal.
One I had the PPL I started hour building and whilst hour building I started the exams distance learning.
Once the exams were out of the way I completed the CPL and ME/IR in the summer. However my CPL couldn't be issued until winter as I couldn't do my night rating until then.
With regards to hour building it makes no difference on paper if you fly a Warrior with all the kit or a basic vintage aircraft. Get the cheapest you can and be disciplined with what you do, ie don't bumble around overhead, go and navigate somewhere. Spanish hours will count, but try and fly in the UK as the weathers always crap and its great experience.
I think, but I could be wrong, that now the retirement age is 65, being 30 when you apply for jobs will not be an issue. Something to consider though is to get an instructors rating and get a few hundred hours. A friend who is over 30 didn't have much luck job hunting so got an instructors rating, had 700 hours then a job on turbo props. He now flys jets 3/4 years after qualifying.
I did an IMC also, but only because I was awarded a bursary by the Air League. Otherwise I couldn't justify the costs. An IMC is great experience and I found it helped with the IR, but only do it if you have spare cash.
GCSE's, A-levels etc will probably not have any impact on your flying and training ability and you'll be just as good as the next person. But you'll be applying to airlines along with hundreds of others with an fATPL and a few hours hour building and the HR dept will find any reason to reduce the number of CV's they have, so try and make yourself as different as possible to everyone else to gain the upper hand. Eg, instructing hours, flying bursary/grant awards, flying something different.
The only other thing I would strongly recommend is to train at a school you feel comfortable at, that way you'll be relaxed and get the most out of your tuition.
You'll need about 32K loan if you work part time and can top it up with these earnings.
Good luck, and enjoy!