A previously cancelled insurance policy is making renewal quotes very expensive. Any advice?

I've previously had an insurance policy cancelled for missed payment and, as a result, I’m having difficulty finding a reasonable renewal quote. What would you suggest?

My son had his telematics insurance cancelled for exceeding a 60mph limit - is this extreme?

My 17 year old son has had a telematics box fitted for the full duration of his two months of driving since he passed his test. His driving has been improving generally with a general reduction of 30-40%...

My insurance was cancelled but I think my black box is faulty - what can I do?

I have had my insurance cancelled for doing 90mph in a 60mph zone. However, I know for a fact I have never sped to such as speed, I have also had no pre-warning of cancellation. Nor have I had any warning...