Probably not what you're thinking of but I actually decided it would be more price effective to cancel my dog's insurance once it got to just under £2000 a year, although, she was slightly older than your dog being 11 or 12 years old at the time, can't remember exactly how long ago it cancelled it.
My reasoning was that paying out £2000 a year before I even started on anything else felt ridiculous, the excess was massive, and then on top of that they took another 20% off the money I got back; the last claim I made was for over £400 and I got a payout from them of exactly half of it. It was so expensive because it involved the out of hours vet.
In addition to that most treatments my dog had didn't even reach the excess amount so couldn't be claimed for anyway.
In the event that she needed a course of very expensive treatment I have enough saving to cover that, which I think is a major consideration if cancelling insurance.
She's now 14 and so far the cost of any treatment she's had would not have reached the excess I had been paying.
I don't know how it is with insurance companies now but it used to be very difficult to change an older dog's insurer as a lot of companies have a cut off point, and those that do take older dog are going to probably charge a very high premium similar to your current insurer. And of course, any previous conditions that a dog has been treated for will be excluded.