Found this on the web:
Exemptions from seat belt wearing
The legislation provides for a number of specific exemptions from the seat belt wearing requirements on medical and other grounds. Please consult your doctor if you think you should not wear a seat belt on medical grounds.
As for the others these include:-
"(b) the driver of or a passenger in a motor vehicle constructed or adapted for carrying goods, while on a journey which does not exceed 50 metres and which is undertaken for the purpose of delivering or collecting any thing;
(c) a person driving a vehicle while performing a manoeuvre which includes reversing;
(d) a qualified driver (within the meaning given by regulation 9 of the Motor Vehicles (Driving Licences) Regulations 1987) who is supervising the holder of a provisional licence (within the meaning of Part III of the Act) while that holder is performing a manoeuvre which includes reversing;
(e) a person by whom, as provided in the Motor Vehicles (Driving Licences) Regulations 1987, a test of competence to drive is being conducted and his wearing a seat belt would endanger himself or any other person;
(f) a person driving or riding in a vehicle while it is being used for fire brigade or police purposes or for carrying a person in lawful custody (a person who is being so carried being included in this exemption);
(g) the driver of—
(i) a licensed taxi while it is being used for seeking hire, or answering a call for hire, or carrying a passenger for hire, or
(ii) a private hire vehicle while it is being used to carry a passenger for hire;
(h) a person riding in a vehicle, being used under a trade licence, for the purpose of investigating or remedying a mechanical fault in the vehicle;
(j) a disabled person who is wearing a disabled person's belt; or
(k) a person riding in a vehicle while it is taking part in a procession organised by or on behalf of the Crown."
Fitting Seat Belts
All new cars must have:
Front and rear seatbelts fitted that must be worn
Older cars that do not have seatbelts:
Do not need to have them fitted and worn
However if seatbelts are fitted in older cars they must be worn