This is from the CAA medical department's own web site, where information about medical standards can be found :
Your visual acuity (measured by your ability to see, in this case, lines of letters on a chart at 6 metres) must be at least 6/9 in each eye separately and 6/6 using both eyes together, with or without glasses or contact lenses (correction). If you need correction the refractive error (the amount of correction) must not exceed +5.00 dioptres of long sight or -6.00 dioptres of short sight.