You can go direct to PPG with BHPA but not all BHPA schools offer PPG training. There's 3 ratings of CP: Tow, Hill and Power that are all at the same level but all preceded by EP.
All schools start with EP but then some schools offer winch training which will lead to CP Tow whilst others offer hill launches which gives CP Hill and some offer power training leading to CP Power, but there's no requirement to do CP Tow or Hill before CP Power. Once you have CP you can do short conversion courses to add to your CP rating. I'm CP Tow, Hill and Power. So you could start with power and go free flying by adding hill or tow later if you wanted to. If you want to go the BHPA route you just need to find a school that offers the CP you want.
I think it's just that a lot of guys start with CP Hill and then become frustrated with the lack of flying opportunities in this country and so convert to power. Which is what I'm in the process of doing. CP Hill with all the kit but now converted to power and on the look out for all the extra kit.