I agree with Dave. Also in the UK credit/amex cards do not have hire car cover as far as I know. So you need to check with amex to see if you would be covered. I rent lots of cars in the UK and always have CDW and theft protection. Hertz offer super protection, but never take it, as it costs more than the car hire.
You can also go to the Hertz web site and change the location to the UK and check out the UK rates etc.