I saw this thread just after I booked and prepaid a UK rental at LHR last month. After I paid for it, I saw a notice that gold service is not available for prepaid rentals. I was ticked, came here, found this thread and others.

HOWEVER, when I got to LHR last week, my name was on the board, the contract was in the car, the envelope said Gold, it showed my original charge and had the voucher credit on it, and I just drove off and showed it all and my license to the gate agent, as usual.

The other unusual thing was that I got a fax after booking with the prepaid voucher details which stated I had to present that when picking up the rental. That annoyed me as well, but I was never asked for the voucher.