My car isnt a UK version but i have one of the top toad alarms with proximity sensor.
Personally I never really use it as I live in a built up area and park it in a car park but I must say they are really good!
Anyway my point is why try to add the alarm to the system, why not just get a replacement alarm with immobiliser, prox sensors, movement sensors and all of the relevent standars that may bring your insurance down for probably less than an add-on?
Just my 2 cents..