It may not be. The excess applies to damage to your own car in the event of an accident if it is judged to be your fault. The insurer should pay 100% of third party claims.
There are two parts to an excess; a compulsory one and a voluntary one. The compulsory one is to stop people making multiple claims for small sums, but some of these excesses can be very high, up to and sometimes more than £1000. You may feel that a slightly dearer policy with a lower excess could make more sense for you. On the other hand, offering a high voluntary excess - which is on top of, not instead of, the compulsory one - may reduce your premium.
However If you make a claim then you will have to pay both the compulsory and voluntary excesses out of your own pocket before the insurer pays the rest. Bear in mind that if you have a cheap car the total of the two excesses may come to more than it's market value meaning that if it was written off you would get nothing back at all from your insurer.
You need to balance the premium reduction that a high excess will gain you against the possibility that you could lose your car with no payout from your insurer. Only you can decide that. Just make sure that you check what the total excess is before accepting what looks like a cheaper quote. You may have to pay it one day so that cheapest policy may prove to be more trouble than it's worth.