no offense, but that insurance sounds like a rip off...
IF you rent a car in the US, the going insurance rate if 15.00 USD per day. However, there ARE alternatives.
A.) If you have auto insurance, you can transfer it to the car you drive in the states.
B.) You can purchase "vacation insurance" for a vehicle through one of the US insurance companies (you could also contact an auto insurance co in the UK & see if they do this, as well). It would be expensive, but not as expensive as that 325.00...
C.) Some Credit Card companies offer auto insurance, so check with your credit card company
D.) If all else fails, here is an online travel insurance company that other people I've known have used:
https://www.travelinsurancecenter.com/eng/
If you HAVEN'T already paid the 325.00 for the insurance, see if you can accept the rental without their additional "special" insurance. Then, when you get to the car rental agency, check THEIR rates, b/c I would be willing to bet the rates are a lot cheaper...
I don't know what you mean when you say you get "third party insurance." If it is in relation to the car rental, the term could mean that the car rental automatically comes with insurance if someone hits you (rather than you hitting them). But even THAT terminology sounds abnormal, to tell you the truth...
Telling you that you need to 'upgrade' your insurance...again, sorry, but it sounds like you are getting ripped off.
If it is any consolation to you whatsoever, had you rented the car on your own, you most likely would have been paying 325.00 for a two week rental, and THEN you would have had to figure out where to get insurance for the car.
My first step, if I were you--check with your credit card company and check with your bank. Tell them you are renting a car on holiday & see if they give you coverage for it.
One tip you need to be aware of: once you are done using the vehicle, MAKE SURE you fill the gas tank back up YOURSELF! If you bring it back to the agency with less than a full tank, the price per gallon of petrol they will charge you is almost DOUBLE what it would have cost you had you done it yourself. And, they don't tell you about the charge--it just ends up appearing on your credit card statement...
Regardless of whether you choose to check out any of these options or just pay the 325.00, make sure you get insurance. Florida is #3 in the nation for auto accidents b/c there are so many different types of drivers running around that drive completely different from one another because they are used to driving in THEIR area. All locations have a "rhythm" to their drivers, and it works for them. But put those different rhythms together and it results in chaotic driving patterns, lol.
Hope this helps!