Depends, if you're young and in your first couple of years driving, that first year's no claims can make a very big difference... so if it's up to £500 or so it might actually work out better financially to give cash, or you end up with a claim on your record for 5 years which over that time will easily add up to more than £500 in increased premiums...
If you're older, or have protected NCD, then may as well go through insurance...