Insurance notification rules for having a tow bar
If fitting a towbar to your vehicle you will need to let your insurance company know because basically it is a modification to the vehicle.
Think about it, this modification does alter the risk assessment of your policy because you will be towing a vehicle caravan, boats, trailer on your car, which can be thought of increasing the risk of an accident.
In general, insurance companies do not make an additional charge but is of course subject to who you are insured with (you may be charged an additional fee because of a change to your policy however but again this is subject who you insure with).
A major reason also to advise your insurance company about your tow bar is so it is covered for damage to the towbar.
If you are shunted from the rear the tow bar may take the full force and actually save the car but it might be damaged in the process. The damage may not be apparent so would need to be sent to the manufacturer to re strength test it.
In reality it is more cost effective to fit a new tow bar so you would need to make sure it is covered in your insurance policy.