A nice mixture would be GCSE English and A/S level Mathematics (or the other way around). I dont think it is a necessary requirement for a career flying, but on the other hand if you have the available time and inclination it will better your prospects. Considering your age, take them part time - it will give you a spur (especially if one of them is taken at a higher level) and will save you missing an opportunity at a later stage in your life when you might not have the time or inclination to sit with the books... for example when you are finally sitting in the rh seat ... :)

Not sure how employers feel, generally for any job they ask for basic qualifications.
If you can pass the ATPLs and with the experience of life, GCSEs should be no hardship.
I did some extra A levels at evening school, 3 hours a week for 30 weeks and passed all
Goodluck
Spitty