From an academic point of view, its seems you have the ability judging by your grades. Your grasp of English Language is a lot better than what mine was when I was 18. The small things like this will matter if you want to be successful at a coveted cadet scheme. So, just by going on your post I would say you will have no trouble with the theory side of things!
When I was 18 and leaving sixth form, I very nearly went straight on to a self sponsored integrated programme. I didn't do it thank God, I can see now that I was nowhere near ready in terms of maturity or life experience. I believe I would have had the ability to pass the course, but my maturity and lack of understanding of the work place would almost certainly have hindered me in getting a job. I'm now 25 with a degree, 8 years work experience and better for it. I'm only now in the middle of a modular training course.
If you have the opportunity to go to Uni, it will only benefit you! Apart from having another certificate to add to the pile, It will further your ability academically and will also give a breadth of life experience. If you have the chance/time to join clubs or organisations at Uni do that as well. CRM is a big part of airline flying now, so if being a good team player and being able to work with others is second nature to you, it will stand you good stead.
If you do go to Uni, I would recommend trying to get a part time job. Firstly, you learn what is expected of you in work i.e. time keeping, personal responsibility - even small mistakes in the workplace have more severe consequence than at school, initiative, endurance, diplomacy etc etc. Secondly, I think if your first ever job is in the righthand seat of a A320/737 there is a risk that you could end up with a jaded view of the working world, I've seen it happen to a lot of people. What I mean is, if you've never did any 'donkey work' in your life then you will have no conception or apreciation of what a good job is. Thirdly, by being in Uni and working at the same time, you will learn how to manage your time as you try to juggle two different responsibilities, excellent experience in my opinion! Lastly, it may help you save some money towards flight training.
If your not minted, I'd stay away from the integrated courses unless you get on to a cadetship. Before you start any training, try to exhaust the cadetships, give them a few shots. Stick to modular otherwise.
You may consider doing an aptitude test just to make sure you have the core skills needed in flying. The wings alliance or GAPAN can do this with you, they will debrief you and tell you what the options are.
In closing, research, research, research aviation and flight training. Ive been reading about these things for years now and still feel like I haven't done enough research. I'm not trying to scare you but there can be a lot of expensive pitfalls in flight training so it pays to do you research.
To answer your original question, you don't need any GCSEs or A Levels to join a lot of airlines, just a fATPL. But, if you want to get into one of the more decent employers then generally you will need 5 GCSEs, two A2 Levels or a 2:2 Level degree as you rightly stated.
Just my two pence!
MP