Its only an intrusion of privacy whilst you are not working.While you are driving around and getting paid for it there is nothing you can do.I am sure there is a company that have been instructed they must not use their system to monitor vans out of hours,can't think who it is though.
The employment rights act does say that if a company wishes to install/monitor (cctv,trackers etc) they have to give good reason.Eg..."we have installed a service monitoring system for our customers...."we have installed it so our vehicle insurance premiums are reduced" etc..