I had a Tracker system fitted about 5 years ago to a brand new Rover 75 Diesel Estate costing £21k but it was a hasty, regretful decision as nobody bothers nicking those motors.
There was also an annual subscription charge in addition at that time and not sure if that still applies.
Tracker can provide you with a list of insurers who give a discount but none were suitable for my needs at that time.
A quality alarm / immobiliser like Thatcham grade is also a good alternative if your motor is not in the favourites list for car thieves and you don't live in a "dodgy" area occupied by lots of joyriders.