1. Do the A-levels. I can't imagine many airlines taking your application seriously without them, and I'm pretty sure many training schools will expect you to have them. Quite a number of pilot applicants have degrees, so where do you think your application without A-levels would go?
2. Modular is a perfectly acceptable training route and in the past this hasn't been a major barrier to getting an airline job. Other personal qualities and aptitude are just as important. Of course OAT and Cabair would tell you otherwise!
3. Believe me the ATPL ground school course and exams are no easy ride. This is borne out by the number of students requiring at least one or two retakes. I can't imagine doing them without education to A-level standard, preferably in science subjects.
4. Do the A-levels!
Good luck