Is it just the third public liability you want cover for? Or rider insurance proper that covers you injury wise?
I've got BHS mainly because I needed it for my exams, but the magazine has the odd good article and discounted tickets to things. No idea re the others.
BHS has the most comprehensive cover I think and the legal helpline. You can also get discounts at events etc...we went for BHS when I compared all the different covers from HH and others