I would try the feliway travel spray in her carrier and the car, and take a warm blanket that smells like home. No cats generally enjoy going to the vets, and I tend to see sedation as a last option. With sedatives, your cat is still frightened, she just can't show it. Thats why I hate the idea of sedating animals for Fireworks Night rather than desensitisation.
Oh, and PLEASE do not drive with cats loose in the car! Believe me, accidents do happen. The calmest cat can get a fright and cause a serious accident. Not to mention the fact that they could bolt when you open the door. Even worse, if you had an accident and the windows smashed, do you really think your cat would stay in the wreckage? It would run across the road and chances are you'd never see it again.
Cats should always travel in a secure carrier, and dogs should either be behind a grill on the backseat or in the boot, or wearing a seatbelt harness. I once saw a mastiff who had been thrown right through the windshield from the backseat in a crash. Please be responsible when it comes to travelling with pets, for their safety as well as yours and other drivers.
Ems