As several posters have said, the "free car" deal is to be avoided, as the additional insurance often costs more than a rental car with insurance would have cost from another source in the first place. That warning has often been mentioned here on TA, although it isn't something people would think to search for. Although at the OP's expense, this is another opportunity to warn travelers to carefully examine the terms of any "free car" offer.
Under Florida law, basic liability insurance to protect others is included on all rentals; it is illegal in Florida to allow a vehicle to be operated on the public roads without that basic protection. However, cover for any damage caused to the vehicle by the renter is a separate issue and is not automatically included unless it is bundled in through a broker or rental car company's international offer. This can be in the form of the rental company's waiver of any damage via a collision damage waiver (CDW) or loss damage waiver (LDW), or by an insurance policy underwritten by a third party.
US and Canadian drivers often don't need the CDW/LDW because their insurance on their personal car is extended to cover damage to the rental car they are driving. Many US credit cards also include cover for such damnage. Therefore, domestic renters really don't want that coverage from the rental company, and wouldn't want the cost included in their rental. On the other hand, international visitors really do need the protection and naturally want it as cheaply as possible; one would think that a travel company like Thomas Cook would accomodate that need as a customer service, even at some reasonable extra cost.
The actuarial cost of providing rental car physical damage cover is probably only a few dollars a day, even considering that visitors unfamilar with the roads are more prone to have accidents. Charging upwards of $10 a day is going to be very profitable, and that's why people are given bonuses to sell it. You don't say how many days the $600 is for, but I'd guess it's likely for two weeks or less.
You could take the approach that you were assured that "everything was included", if you can prove that was said. The counterargument is that the physical damage protection is technically optional, even though most sane non-billionaires wouldn't drive without it.
Let us know if you find a way to avoid paying the full amount, or even part of that high cost. Good luck.