Most folks begin on FW and go on to get their rotary licence later
I'm not sure that this is the case. When I was doing my PPL(H), most of the other students had started straight off with helicopters. Similarly, as an instructor, I don't often get f/w pilots; it's usually people who just want to learn to fly helicopters. And considering the CAA only gives you 6 hours off the course if you've got a PPL(A), and not many people do the PPL(H) in minimum hours anyway, I'm not sure that it would save you any money...though extra experience, in any flying machine, is always useful of course.