MSE has a good article on getting a good deal:
http://www.moneysavingexpert.com/travel/cheap-car-hire
* Collision damage waiver (CDW).
This is the basic cover and it reduces your liability in the event of an accident. Without it, you could be forced to pay for the replacement cost of the vehicle.
* Theft waiver (TW).
The equivalent of CDW but protects against theft. Also known as theft protection.
* Super collision damage waiver (SCDW).
Reduces your liability to zero and covers tyres, roofs and windscreens.
* Personal insurance (PI).
Pays out if you kill or injure yourself or a passenger, usually covered on regular travel insurance.
* Personal effects cover (PEC).
Pays out if your property is stolen from a vehicle, usually covered on travel insurance.
Holiday Autos sometimes give free SCDW or you can buy it seperately (links in the MSE article)