I had to have a CAT 1 alarm/immob fitted in order to get insurance. It was a Cobra system and was £240 fitted. Fairly basic, couple of sensors inside and one on bonnet and boot.
Had it fitted by a 'professional' shop in Hemel because I needed the fitting certificate. They did an OK job but got arsey when I asked for all connections to be soldered because I didn't want any electric connectors to be used. Seems to work fine although my battery goes dead after 2 weeks, seems to be the norm though. I now have a mains battery conditioner/charger that keeps the battery topped up, works a treat.