Voluntary excess This is chosen by you and is in addition to any compulsory excess. You can set this as low as zero to reduce how much you’d have to pay out in the event of a claim, or you can increase it which may reduce your premium. You’d pay the amount you’ve chosen in the event of any claim.
Compulsory excess This is set by the insurer underwriting your policy.
The compulsory excess is added to the voluntary excess to you give the total excess you would pay out in the event of a claim. For example, if you choose a voluntary excess of £350 and a compulsory excess of £100 applies, the total excess you'd pay if you had to claim would be £450.