Well let's put you in the picture on this with a bit of a history lesson.  Way back in the good ole days  there was a grant available which was roughly £700.  Only certain vehicles were eligible and the kit supplier bumped up the price of the kit to cover the cost of developing their equipment for best emissions on a particular vehicle.  The installer also increased their costs somewhat.  
The good news is that you can now get a car converted with a better system for about  £100 more but without the red tape or special conditions (if you received a grant and sold the vehicle within 3 years then you had to give a percentage of the grant back! If you only kept the vehicle  for 2 years then you had to give a 1/3rd of the grant back)
The grant days are gone but the systems are better and cheaper.