Some answers:
I don't believe the interim system is available (in the UK) anymore - you have to do the heli-specific one if you want a CPL(H). Which is fine, as it's much better than the interim one anyway.
You can subscribe to the Bristol questionbank whether you're a student of theirs or not. Last I looked, it cost £50 for 3 months of access.
My recommendation would be CAPT for the groundschool, with a subscription to the Bristol QB for final brush-up before exams. This is the route I followed, and it worked amazingly well.