I use the AL-Ko hitchlock on my caravan, and I have never had any problems getting insurance. I believe that Bulldog now make a lock for the AKS 2004/3004 and this is also insurance approved. I read a review on it that it was a good lock, but they couldn't understand why bulldog had made it when Al-Ko own was so good. The bulldog was also the same price RRP.
Another quick point, make sure that you do NOT buy one from a auction website, I read (on this very forum I believe) of someone who bought one from an auction web site and then got his caravan stolen, as the original seller kept a spare key and had the address where the lock had been dispatched too...