Hertz Gold Rewards has redemptions in Europe but, sadly, they are generally limited to manual transmission vehicles. There are threads in the Hertz forum about this annoying restriction.

When renting a car overseas, don't forget to look at the UK version of carrentals.com and kayak.com (google them with a "uk" added). Sometimes the rates are cheaper when bought in the US, sometimes cheaper when bought in the UK.

Whatever the best rate you find, call Auto Europe and see if they will beat it. They usually will, and that gives you a travel agent to help if something goes wrong (and it sometimes does when renting cars in weird places).