I just checked again on that, its the passenger that is responsible if over 14:
The law on wearing seat belts in the UK
and
Adults
Drivers and adult front seat passengers in cars must wear a seat belt, unless they have a medical exemption certificate. Adults travelling in the rear of a car must also use seat belts, if they are fitted. It is the responsibility of the adult passenger (not the driver) to ensure that they are using the seat belt.
RoSPA : Road Safety : Seat Belt Law