Toughy this one.
If you want to apply for sponsorships, when they start again, I think you will need the A Levels as 2 A levels at grade C and above seem to be the normal minimum criteria.
Ironically the B-TEC that you want to study would probably help you more with the ATPL exams as it is a related course.
I can't really offer anymore help, I guess it is a difficult decision that you have to make. Personally I would opt for the A Levels, they seem to carry more weight out there in the big wide world, why? I honestly don't know.
Sagey