This document from the army is currently in use (as of May 2015).
It explains that the minimum requirements for enlistment include literacy and numeracy skills at Entry Level 2, which is equivalent to the reading age of a 7-8 year old (page 12).
It also shows that young people enlisting aged 16-16.5 years can only join frontline combat roles (infantry, armoured corps, artillery) or as drivers in the logistics corps. Those aged between 16 and 16, 3 months, can only join the combat roles, which in most cases is the infantry (see page 8). For obvious reasons, combat roles are far more dangerous than most other jobs in the armed forces. For example, the British infantry’s fatality rate in Afghanistan has been six times that in the rest of the army and seven times that in the rest of the armed forces. This means that the army’s very youngest recruits face a greater long-term risk, on average, than do older recruits.
Download this guide here.