British Rowing is the official governing body in England for rowing.
For details on the law regarding towing boats or carrying them on a car roof see the following links.
Coxing and rowing on the Tideway
There is a video guide which offers tips and advice on coxing Head Races on the Tideway and is well worth watching.
Also worth watching is this British Rowing video which gives details on navigation rules and how to stay safe on the tidal Thames.