Virtually every pet insurance has an excess, so you have to weigh up the pros and cons. If you have the means to pay a large vet bill should one come up, then I would say put the money away each month and it would be there should you need it. However, if you could not raise the money for a large bill should one come up before you have saved enough, then take out insurance. In over 40 years of owning many animals the only large bill I have had was for �250. Had I paid premiums every month I would have paid a fortune (but had I put it away I would be a millionaire by now!) .
A lot depends on the breed, for instance a big breed like a Rottie is very susceptible to cruciate ligament problems and might need an op costing �2,000. Cavaliers do often have heart problems, but that is usually dealt with by an ongoing course of tablets, rather than a big op. Some insurance companies only pay for the current year, some for ongoing conditions. Some have a limit of hour much per condition, some have no limit. Try googling 'compare pet insurance' and you will get sites like money supermarket.com which will list the different insurances available.