Wilco,
Yes, I did my PPL(H) a few years back, after 30 years with a PPL(A), then did my CPL(A). CPL(H) is a ways off yet$$.
I can't speak with any knowledge about the UK rules, but In Canada, I got many hours of flying credit toward PPL(H) from PP(A), and only had to sit one 30 question differences exam for Helicopters.
Do your PPL(A) first, so you're not paying the big bucks to be airborne, just to learn which way is up - do that initial flying on something with a low hourly cost.
The two types of flying are surprizingly different, but both very worth learning well. Each will give you respect for the other! I went on to do advanced role training in MD500D's, and was amazed not only at what the helicopter could do, but what I could make it do with my modest skill!
I highly recommend training in Schweizer 300. My choice was not the R22 (I had flown both). After you have 20 hours fixed wing, go threat yourself to an hour in the S300. Ask to have an autorotation demonstrated to you. Fixed wing forced approaches will seem very gentle, with lots of time, after that!
Good luck!