Many sex offenders don't receive any form of 'treatment' until after they leave prison. They may then be required, as a licence condition set by by the Probation Service (or through a court order made at the time of sentencing) to participate in the 'Thames Valley Project' (or similar) . However such programmes can usually only take place for a few hours each week. (That's partly due to allowing ex-offenders to re-start their lives by taking on employment, but probably just as much to do with the Probation Service's time and staffing constraints).
Starting 'treatment' at an earlier stage (i.e. in prison) is more likely to be effective since it starts closer to the offence and, more importantly, it can be far more intensive. It's such a policy which HM Prison Service (together with other relevant agencies) is seeking to use at HMP Maidstone.
Chris