Excesses only matter when a car is involved in an incident and needs to be repaired. At this point they really matter though, making the financial strain of repairs somebody else's concern. Cheaper insurance policies can offer amazingly cheap voluntary excesses, even down to paying nothing whatsoever. Unfortunately, the bottom line here is the compulsory excess which can go up to £3000+. The purpose of this is clear- your car will need more than £3000 worth of work to be done or you won't claim for it. So, unless you are willing to have your car out of service until you can afford to pay for repairs yourself, making sure the excess on your policy is affordable is a must.
(Lack of) Customer Service
The last thing you want to be doing when dealing with in the aftermath of an accident is an automaton where you are unable to get yourself heard. Yet this is what has happened to customers of cheaper policies who- having phoned up twice to be told that the company is experiencing high demand and will call back as soon as possible- were still waiting to speak to someone simply to report their incident after 48 hours. Some comparison sites rate the insurance companies they deal with and it is worth using this to assess how other drivers have found their insurers to perform.
Patched or Repaired?
Who repairs your car and who is liable if this work needs repairing again is another point to think about. A lot of insurers want your vehicle to be repaired by their approved mechanics, meaning they are responsible if things aren't as solid as they should be. It may not seem a big deal if a cheaper policy allows you to make your own arrangements- but if these repairs quickly need repairing you are liable instead. Make sure you are clued up about your options before you buy your insurance.