My own experience is that in those establishments where participation is purely voluntary, prison education provides a lifeline for prisoners whose schooling has been disrupted or ended early (usually because of exclusion). I've helped support adults in their 40s, 50s or even 60s learn to read and it has been a very rewarding experience, sometimes even more so than my previous career prior to imprisonment.
I think it's necessary to make a clear distinction between encouragement and compulsion. Locking an adult (or a child for that matter) in a bare concrete box for 23 hours a day because he or she struggles with reading, particularly in a classroom setting, is utterly counter productive and undermines everything good teaching practice stands for.
Many of the prisoners I helped to improve their levels of literacy had experienced horrendous abuse as children, some in a school environment (boarding schools, 'special' schools, approved schools). Forcing them back into a classroom as very damaged adults - as opposed to offering one to one tuition, at least until they had regained confidence in their own abilities to learn - was unimaginably cruel and led to at least one suicide of a prisoner who simply couldn't cope with this pressure.
I fear that this policy of compelling prisoners to attend education classes or face punishment erects a final barrier that will discourage many from learning for the rest of their lives. What a tragic waste of resources and opportunity.
The other major problem is that recent budget cuts mean that no courses above Level 2 (GSCE equivalent) can be offered by prison education departments. Any higher level courses have now to be funded by the prisoner and recent changes to the IEP system (PSI 30/2013) have effectively blocked the sending in of course books or other material. This is having a very negative impact on those prisoners who really want to improve their education and gain qualifications, whether academic or vocational. This merely undermines attempts to achieve rehabilitation. Sometimes it really does seem that the Ministry of Justice and the National Offender Management Service (NOMS) are just setting prisoners up to fail.