Security deposits are always pre-authorized but you usually can't use them for damage. I don't know about British laws but there are certain states within the US that limit a renter's responsibility but Hertz doesn't authorize to that amount. The deposits are mainly used for late charges or fees not relating to damage. I would charge people immediately for lost keys, towing charges, impound fees, etc... and those could be easily be $200-$500 but I couldn't charge $50 for a flat tire unless I had that person's signature admitting the damage and agreeing to pay up front.