Depends on what your commitments are. But if you get your PPL at 17, you could be ready to do your CPLs skills test at 18. I don't know the exact rules, but you only need to be 18 for CPL issue or maybe test.
I suggest you apply to the Bristol Ground School ATPL scholarship where you'll get free ATPL theory if you're one of the winners.
This if for modular. The better route IMO, but I am biased and others will have their own opinion.
Join the cadets if you can, go gliding.