That's interesting, I don't think I would have expected literacy to be much of an issue in the UK.  In most of Africa, including here, each party or candidate has a symbol that appears on both the ballots and the campaign posters, so one need not be able to read in order to mark one's ballot.  In some countries, the ballots also have a picture of the candidate, and/or a hand signal associated with the party.  (Example: in Zimbabwe, the ruling Zanu-PF party is a clenched fist, while the opposition MDC is an open hand.)  Those hand signals are also represented on the ballots.

In Jordan, the parliamentary ballots ALL seem to require that the voters write in the name of the candidate, there are no boxes to check, or at least there weren't in 2003.  I was puzzled by this and asked my colleague, "What about people who can't write?"  And she looked at me like I'd lost my mind:  "Nobody can't write."  Which is not actually true (Jordan's overall adult literacy rate is around 90 percent) but probably is fairly close to true in the urban areas and among the younger parts of the population.