If you become a Supporter Member of British Show Jumping www.britishshowjumping.co.uk it only costs £28.00 and this covers you for public and personal liability for any horse that you ride.  You also get all the other benefits of being a member and get some free stuff!

Become a Gold member of the BHS, it includes insurance when you are riding horses of your own and others in general.

Make sure that the horses you ride are covered themselves for public liabilty as well. I suggest if its for a buisness your public liabilty cover should be at least 3 million pounds.

Otherwise your looking at around £200-250 a year to insure your services as a whole as a groom which includes riding (but make sure that all the activities are covered). My insurance was £204 a year but didn't include hunting or XC competitively.