With the BHPA, and to fly solo, you need to have a Paraglider CP rating and/or Powered Pilot Rating to get their insurance. If you have passed both, then there is no extra charge. BHPA flying membership costs £89 (£7 less if paid by direct debit) annually and includes subscription to Skywings (the monthly magazine of the BHPA) and £2 million 3rd-party insurance cover. There are concessions for under 16s, 21 and under, and over 60s.
If you only want powered cover, then there are cheaper options out there and you dont need to pass any thing to get the insurance.
Cheers
Mike