Cornwall is expensive as we know! Anytime of the year! Only thing is I am in love with Cornwall. We are off to Ireland for a week, cost £550 which includes our apartment and return ferry. We spent more than that on a cottage in Weymouth in June this year for a week..
Don't know about in car cool boxes. My hubby uses an Igloo cool box. They are brill. We use our big one when we go to Ireland. It will keep food frozen for at least twelve hours or more with ice blocks if need to.