I have also booked a fly-drive with Direct Holidays for Oct.
At the time of booking I declined both their travel and car insurances. This was not a problem for the agent I was talking to, although she does require that I send a copy of our
travel insurance documents before we go (to make
sure it's adequate which is fair enough).
As for the car insurance, I explained that I would get this elsewhere (I was thinking of using insurance4carhire but didn't mention the exact compny name) and she said that was OK. In fact, I doubt that the hire company in Orlando would care really. Certainly they would not be getting (and therefor losing out on) the full £364 that Direct Holidays are charging. Most of their customers will pay full whack, so the few who have done it independently
might get a bit of hard sell, but I am certain that they would not refuse cover from elsewhere.
I do have some concern in not using the insurances offered, but I also hate getting ripped off (and £300 for travel insurance [I have paid £52!], and £364 for car insurance [I expect to pay £99 + local taxes], is a rip off.
There have been many other threads regarding insurances - it might be worth doing a search to get even more opinions.