Are you sure?
they are taking on another £10k of liability to insure griff's 'van over yours. That liability comes at a price to cover.
Not quite. The liability for the full value of the van only kicks in if there's a total loss of the van (either via accident or theft). That reflects only a portion of their liability, e.g. they could be covering damage to the van via accident, or theft of contents. Think about your car insurance - the cost doesn't increase linearly with value of the car (having 3 vehicles in the household with identical drivers I'm well aware of that one). I don't know the underwriting criteria used with this particular quote, but I'd expect perhaps a 10% differential for the higher value insured.
I can say categorically that the underwriter with CCC (and I know other people have got different answers from CCC) said if I only fitted one lock, they'd treat it as no locks fitted as it's chocolate teapot territory (their words).