You do tend to get what you pay for with it - the cheaper policies sound ok, but either do everything they can to get out of actually paying out, or don't provide that much cover. It's a balance of finding one with good cover, but one with premiums you can afford to keep up.
Make sure to get one that covers illness as well, some of the cheapest policies are "third party" only (so if your dog attacks someone/another dog), or "accident only" (which excludes things like skin problems, diabetes, heart problems etc. It only covers an actual 'accident', so a broken leg etc).
Two main things to look at are the amount of money cover, and lifetime cover.
Lifetime cover means you pay your policy renewal each year still, but things won't be excluded if they have been diagnosed before a claim needed to be made, or for long term illnesses they will keep paying out. Examples of this are things like diabetes, a yearly only policy means you can claim for one year and then anything relating to that will be excluded in future years. Also, things like diagnosis of a heart murmur this year, which may not need treating yet. But if it does then need treatment in a years time, a yearly policy will have excluded it - despite no claim being made. This is the biggest difference between costs, lifetime cover obviously being more expensive than yearly. Yearly policies are ok for 'emergency' type things (a one off fracture etc), but anything lasting longer is limited cover.
The other biggest variable between policies is cover limit. Some are a limit per year, some per condition/illness. I would say £5000 is the minimum cover you should want, ideally more like £7000-8000. Referral for an MRI scan would easily be £2500-3000, and then surgery/treatment on top.
Pet plan is far and away the best insurance provider. They pay out the quickest, quibble over things the least and have good cover amounts. But this means they do tend to be expensive. Agria (Kennel Club) are good as well. Quite a lot of the supermarket ones are also not bad, but look at the level of cover provided. (Often there are bronze/silver/gold options etc, with higher options having better cover). Protect your bubble, animal friends and anything underwritten by E and L are generally not great.
Bear in mind as well that you can't easily change insurance companies once your dog has had any problems. So try and go for one with a premium you can afford without having to change. As soon as a vet had seen your dog for a lameness/skin problem/heart murmur etc, a new insurance company will likely exclude anything previously noted.
Have a look on this website as the reviews on there should give you a bit of an idea of which ones are better than others. www.petinsurancereview.co.uk/dog.asp