Did you ask them about PL specifically? When I tried to claim on Dds rider insurance for this, I was told that I was liable, not her as she was a minor. It might be worth asking them the question specifically, just to be clear.
If you're full loaning do you need to insure for vet and loss of animal, or is the owner retaining this? If you need cover I can recomend NFU Mutual, but if just public liability then Pony Club or BHS should be fine
I was going to say BhS. You get a free legal helpline, and a certain amount of personal injury. (Also, once a year in their magazine, they publish a free ticket to Hickstead. If you're both members, and you ring and ask, they'll send you a spare mag, then you and dd get a free day out! )