Insurers usually require that it's a Thatcham approved device, and you'll be asked to prove it, so keep the installation certificate handy. For some expensive cars, insurance companies may require you to have an alarm, but it'll usually qualify you for a discount. Remember, if you do opt for the immobiliser discount, you need to make sure that it's in operation every time you leave your car. If your car is broken into and your alarm wasn't switched on, the insurance company may not pay out. Where you park your car overnight has a strong impact on your premium.
Insurers rate different postcodes around the country by their estimate of the risk in that area.
Insurers also prefer you to park your car in a drive or garage rather than on the road. Try some different options and see what the effect is on your quote. Generally, the more people driving your car, the higher your premium. It's better to decide who will be allowed to drive and choose an insured and named policy. If you want to leave the car insurance open for anyone to drive, you can opt for an any driver policy.