How much is the cover for? I.e. what is the maximum payout and over what term? It’s usually better to go with the dealer, you might pay more but if you have to claim and the insurer gets shitty then you can get the dealer to argue your case. The internet won’t do that for you.
I cannot say get gap enough, I had my 6 month old car stolen on new years eve and the difference between the insurance payout and what the car cost new was £7500. Thankfully all covered by the gap insurance.
I paid £399 for a 4 year policy that would pay out up to £20,000 at any time during that 4 year period.