Bulletzone...
Firstly to whoever said there's a low demand for pilots at the moment, in the US - yes; In the UK? Maybe but I don't think so; In the world? Hell no there is not! There is an extremely high demand for pilots! it is one of the fastest growing industries and it is expected to need more pilots than are currently being trained at current rates. This is the main reason courses like the integrated ATPL and MPL exist - to fast-track cadets into the right hand seats! Anyway I digress...
As a pilot in training, I assure you the qualification subject does not matter as much as the qualification itself. You can probably be a pilot even with A Levels such as Hair & Beauty (don't quote me) but if you don't have the A Levels then, obviously, you can't. So do the subjects that you enjoy, and that you are good at...because then you have a better chance of getting them! Of course if Maths and Physics are your strong points, take them. Maths is the best to take (mainly because if you're good at Maths you're less likely to make a critical mistake in your calculations) and a good understanding of Physics is very helpful too. Things like Chemistry, Geography and Languages aren't really going to help you that much to be honest (I did languages to degree level) and even engineering, which you'd think would help, doesn't help a lot. Maybe in one subject at ground school. But you want to fly the plane, not build it. But anyway, they're all good subjects. Just do the A-Levels you enjoy the most and think you can do best in. That's more important than anything else.
As for a degree...it is true that it is becoming more common for cadets to go straight into training after their A-Levels. However a cadet with 3, or even 4 A-Levels will, 9 times out of 10, not be picked over someone with a degree, provided they have a similar flying background. A degree is preferential to most airlines, but like people have said above it is not compulsory. For some airlines it is. I think for easyJet, it might be compulsory to have one but I'm not 100% sure. By all means, apply after A-levels, I have friends here who did and are now on a sponsored scheme with me to work for a major Middle-Eastern Airline.
Of course the other way to go is the RAF / Navy / AAC (Army Air Corps). I don't know about the Navy or AAC but I applied for the RAF and passed the tests to become a pilot...and then I found out from another RAF pilot friend of mine that it would take about 6 years or so training. I wanted to be on fast jets so...1 year basic training, 1 year Initial Officer training, 2 years basic flying training then specialisation, which was another year for helicopters and I think 2 years for transport or fast jets. Next thing he said was he was glad he got onto transport and not fast jets, because those guys are lucky if they fly twice a week! In comparison, I will have completed my training within 2 years of starting and be flying the A320. Granted, flying a fast jet would be more thrilling, but you'd definitely fly much less (more pilots than planes apparently).
So I'm glad I didn't join the RAF - going to be able to afford a good living because of it! And I'll fly more than twice a week (which is what I want!), I took languages and did well in my A-levels and got a degree, which is not necessary, but helps.
Anyway, whatever you decide, enjoy! You'll still have an office at 40,000 ft and you'll love it! All the best and if you want any more info, I'll do my best to help.