Posted By Bryan Nott
I am not sure whether there is any specific guidance but the Workplace (Health, Safety & Welfare) Regulations 1992 regulation 17 covers the generality of it. I think it would be very difficult to set a standard 'width' but I may be wrong.

Organisation etc. of traffic routes
17.—(1) Every workplace shall be organised in such a way that pedestrians and vehicles can circulate in a safe manner.

(2) Traffic routes in a workplace shall be suitable for the persons or vehicles using them, sufficient in number, in suitable positions and of sufficient size.

(3) Without prejudice to the generality of paragraph (2), traffic routes shall not satisfy the requirements of that paragraph unless suitable measures are taken to ensure that—
(a) pedestrians or, as the case may be, vehicles may use a traffic route without causing danger to the health or safety of persons at work near it;
(b) there is sufficient separation of any traffic route for vehicles from doors or gates or from traffic routes for pedestrians which lead onto it; and
(c) where vehicles and pedestrians use the same traffic route, there is sufficient separation between them.

(4) All traffic routes shall be suitably indicated where necessary for reasons of health or safety.

(5) Paragraph (2) shall apply so far as is reasonably practicable, to a workplace which is not a new workplace, a modification, an extension or a conversion.