In the Paras you can wear glasses/contacts and you only need to be 6/12 acuity in ONE eye CORRECTED(!!) for entry/retention (In the other you only need to be able to see the top letter on the vision (Snellen)chart = 6/60 i.e blind as a bat!!). Rare for someone to be refused entry to the paras on visual standards alone.
You CANNOT wear glasses in the UKSF!
If there are guys wearing glasses going for 21/23 then if they pass they will not be allowed to wear them and must replace them with contacts.