The voluntary excess is the extra amount, over and above the compulsory excess, that is agreed to be paid in the event of a claim on the policy. (en.wikipedia.org)
Choosing a higher deductible reduces the amount of money your insurance company pays in the event of a claim, so if you're willing to pay a higher deductible, insurance providers are willing to lower your premium. (thefreefinancialadvisor.com)
Increase your excess to reduce premiums — but make sure you can afford to pay it in the event of a claim. (moneysupermarket.com)