I have had joy from holidayautos.co.uk....it is variable, but often lets you get a Hertz or Avis car for say E100 not E300, you just print out a voucher and present it to the desk at the airport. In the UK it seems to be a problem, the hire companies try to add on 17 different flavours of insurances and  waivers to recoup the discount you have. Over here I once paid an extra Euro for zero excess and was glad, after a little roll Avis' Clio was a "slightly distressed" and I dodged a large bill....