Hi there - I know Cashplus and also ClearCash allow for car rental - cashplus you will need to call up though to inform them - with both you need to make sure you hace sufficient funds to 1. cover the rental cost and 2. cover the hold rental companies put on their clients cards and then release upon return

This recent thread from the other day may help.

http://www.moneysupermarket.com/community/forums/t/pre-paid-credit-cards-39279.aspx