I suspect that the real reason why they are let out early is that we now have far more convicted criminals awaiting sentence than available prison places to put them in, so we have to keep letting them out early in order to be able to shove the next lot in.
We should bring Dartmoor Prison back into service and build another similar one alongside it -and at the same time re-introduce penal servitude, or `hard labour` as it was known; 4 or 5 years of breaking rocks all day and every day in all weathers, miles from anywhere and under the watchful eyes of guards armed with loaded rifles to discourage potential escapees, was far better encouragement to "go straight" than all the nice kind Aid Societies, Probation Officers and general do-gooders who are now supposed to watch over the second half of the term to which the prisoner was sentenced. There weren`t many convicts who served 2 terms "on The Moor".