Its mixed reviews on the forum, but to me depends on the task being carried out

HSE says "Site operators and employers must make sure that workers are wearing seat belts when necessary, and are not taking risks that could cause vehicles to overturn."

http://www.hse.gov.uk/workplacetransport/information/overturns.htm

 But Publication HSG 76 states

Operator restraint

391 Operator restraint should be worn at all times when lift trucks are in motion unless, subject to risk assessment: ■ the lift truck operator needs to dismount repeatedly and frequently (eg to position loads on the forks or check stock levels); ■ the truck is used on a smooth, firm, level surface (eg concrete floor); or ​​​​​​​■ the truck is unlikely to be operated at speeds or in ways that could cause overturn due to the nature of the operations being carried out and the area in which it is working.

http://www.hse.gov.uk/pUbns/priced/hsg76.pdf

It's partly to do with the mousetrap effect - a seatbelt must be worn if the FLT has overhead protection, so that if it tips over, the operator is held in place and doesn't get hit by the canopy