Perhaps the answer here is to insist that a renter must either have valid insurance already that would pay out to third parties in the event that they were driving any vehicle. Most fully comprehensive insurance in the UK already covers this assuming you have valid fully comp on some car already. For example I (being fully insured on my own car) could drive a friend's car without being specifically insured on it and…
> insist that a renter...have valid insurance [or pay a higher fee] If you don't own a car it is just about impossible to buy a liability insurance policy. I know this. I tried. I tried very hard. I don't own a car myself, but since I often rent a car when I travel, I decided that I'd try to buy a third-party liability policy instead of paying the absurd prices that Avis or Dollar charge (>$20 a day). I called about…
One of my contracts once required that I have $1 million in insurance company to work for them, including coverage for any employees traveling related to the work. That was 10 years ago though, and 1) I don't recall if the insurance included liability insurance, and 2) I think it only covered travel related to business, so while an odd-ball thought, it probably won't work for you.
What do the people in the NE (NYC, Boston) do? I thought that quite a few don't have cars but instead rent them for trips or for moving. Surely a number of them are in the same boat.
It seems that non-owner liability policies do exist; so says http://www.caranddriver.com/reviews/do-i-need-insurance-to-r... . For that matter, GEICO mentions the possibility at http://www.geico.com/information/aboutinsurance/auto/insight... .
And http://www.carsdirect.com/car-insurance/can-i-get-car-insura... "Every insurance provider is required to offer non owner car insurance, so you shouldn't have a difficult time finding this insurance if you need it. Well known providers such as State Farm, GEICO or Progressive are providers you might consider. Some banks offer auto insurance, so those that do will also be able to offer non owner car insurance." ... "Non owners insurance quotes are available from all of the major insurance companies. Progressive, Geico, State Farm, and Nationwide are just a few insurance companies that offer insurance coverage to people who do not own cars. You can also get quotes from insurance consolidators like esurance or netQuote."