Even if you can get it the thing is with all insurance the insurance companies make money. Thats why most big companies self insure.
For example Ladbrokes bookies have no insurance against robbery. They expect to get robbed every week but the cost of the insurance is more than the losses.
Have you considered controlling the temperature of your stock instead of insuring against melting? A/C in vehicle and stock room. Obviously, apart from placing towers away from windows etc, you can't do much about the stock on site. You can have a condenser-less A/C unit installed in small room for about £1000 or purchase portable unit for just an few hundred quid! Make sure the AC works in your vehicle or just avoid visiting sites on the handful of really hot days we actually get in this country.