In every Hertz booking you can read the following warning:

This reservation is based on information you have provided Hertz, including information regarding your current geographic location.
Hertz reserves the right to demand proof of what you have told us. In particular, if you have indicated that you are located
outside your country of residence, you may, at the time of rental, be required to prove where you were when this reservation was
made, through evidence such as an air or rail carrier ticket receipt, a hotel bill or a utility bill addressed to you at the
location where you say you were. Failure to provide such substantiation may invalidate the rates you have been quoted, or
invalidate the entire reservation. Providing false information to Hertz in connection with this reservation would also breach your
rental agreement and could violate the law.

I've personally noticed that on Autoeurope site the best rates provide if you set your country of residence either UK or Ireland. But even if you have another citizenship (USA etc.) you can use two links - one and two - for skipping the initial question about your country of residence if you go direct to http://www.auto-europe.co.uk and http://www.autoeurope.ie/ accordantly. Tested hundreds times personally and by friends - no problem + best price on the market (note: they also offer best price guarantee).