Road Traffic Act 1988 (c. 52) (http://www.opsi.gov.uk/acts/acts1988/Ukpga_19880052_en_1)
Anyone understand if so tell when u do not have to wear seatbelts

He users of vehicles constructed or adapted for the delivery of goods or mail to consumers or addresses, as the case may be, while engaged in making local rounds of deliveries,

---Mail & delivery people that make frequent stops.

(ii) the drivers of vehicles while performing a manoeuvre which includes reversing,

---guessing that it means if the seat belt is restricting your ability to back up safely you can undo it until you are finished reversing.

(iii) any person holding a valid certificate signed by a medical practitioner to the effect that it is inadvisable on medical grounds for him to wear a seat belt,

---if you are pregnant or have a medical condition that a seat belt would complicate

(c) may make any prescribed exceptions subject to such conditions as may be prescribed, and
May prescribe cases in which a fee of a prescribed amount may be charged on an application for any certificate required as a condition of any prescribed exception.

---The State can require you to pay a fee for proof that you have a medical condition or have a job that excludes you from having to wear a seat belt.

It is from the OP's link
Wish I could say I found it.

The ---stuff was my interpretation additions though :D