Bear in mind that a lot of UK car hire companies forbid the vehicle from leaving the UK.
Also, check out whether full insurance is included in the price or whether its an added extra payable at pickup. Sometimes the excess can be £500. Hertz are one of those that have a high excess unless you buy their additional insurance, which can be £20+ per day.
An alternative is to buy an annual car hire insurance policy that covers you from the high excesses. I use:
http://www.insurance4carhire.com/