Insurance is a personal thing because it's all about providing the cushion you need, just in case - and what you need might vary depending on how much risk you want to carry yourself and/or your access to savings if you get a very high or longterm bill.
I did not want to carry the risk of paying out for long term or large medicial bills so I'd got the highest level of cover I could, thinking that it would be those types of bills that would cripple me. I am currently in a position of having paid around £1100 on insurance premiums but having claimed £10,500. I am forever grateful I chose the maximum insurance level I could find.
For the same reason, I'd look for something that covers issues for life, rather than have a limit per condition. That way, cost will not be a factor in choosing what healthcare options are best for the dog.
Patterdales tend to be pretty healthy dogs but I've also had a similar terrier that required several £000s surgery following a horrendous dog attack, so healthy doesn't mean no risk.
PetPlan remains a very popular choice (and pays the vet direct so you don't have to find the cash and then claim it back) with a high reputation.
BoughtByMany also seems to be becoming very well-regarded and has high cover levels.
AnimalFriends continually gets slated on forums.