The correct and valid authority to contact on untaxed cars on public highway is the DVLA, if a car is taxed (but not moved for a while) there’s little the authorities can do as its taxed and is allowed to be parked safely on the highway.
Watford Borough Council will get involved if the vehicle is causing issues such as ASB, badly damaged, burnt out etc and the police will get involved likewise for similar issues plus if it’s causing highway safety issues.
These might be of use: