I can't find the detailed medical requirements on the new atcocareers site, but the vision part is broadly similar to the JAR Class One medical initial requirements which are detailed here:
http://www.caa.co.uk/docs/49/SRG_MED...%5B2189%5D.pdf
The UK CAA Class One medical that is required for an ATCO licence may be slightly different, but the standards detailed above will give you a rough idea.