The enquirer is a Contracts Manager, who attends construction sites but does not get involved in day to day site operations. He is being advised that he must either provide himself, or his company provide him with bespoke safety glasses (which would need to be varifocal) to wear on a particular site, where they are imposing a blanket requirement on the use of all PPE equipment ie Hard Hats, Hi Vis Vests, Safety Footwear, Gloves etc.
Health and safety law does not prescribe when and where safety glasses must be worn, but would expect protective equipment to be required only in areas where there is a real risk. Many sites do choose to set PPE as a "blanket" requirement rather than specifying when and where it should be worn. The site should be prepared to explain the reasons behind their particular PPE requirements.
However, insisting on the expense of prescription safety glasses for an occasional site visitor does seem unreasonable when there are a variety of overglasses and goggles available on the market which should provide suitable protection for someone who wears prescription lenses and is an occasional visitor to site.