the legalised gambling about which you speak is a major ripoff. The variance in pricing can vary through the day depending upon the targets that the person has to hit.
It certainly pays to shop round but I would rather use a decent broker that you can sit in front of than wait forever on the phone, it costs a little more maybe - but I know that I can always rant at the same person!
BUT - bear this in mind, if you use a phone based service and they neglect to ask you the correct questions to which you are obliged answer correctly -You may not be insured! a lot of people don't read the policy documents and just assume they're covered - and the companies WILL look for ways not to pay your damages.
You lie about job, address, penalty points - non disclosure - no cover.
These pearls of wisdom apply to all insurances, not just your cars. Classic case in point - travel insurance, have you ever had? and you forget to mention it cause it was so long ago - then you have a relapse, get stuck abroad....non-disclosure, no cover - HUGE bill - stuck!