"He has basically been sentenced to 34 months with the option for it to be increased,"
"Instead of shortening the sentence for good behaviour, ..."
"It's so there is an incentive to behave well inside prison Daisy. "
It's not actually that complicated but there seems to be some serious misunderstanding here, particularly linking release at the half way point with "good behaviour". With a few exceptions (see below) there is no such link. Almost all prisoners serving determinate sentences are released having served half of their time. The release is automatic (they do not have to "apply" for it) and unconditional (behaviour, good or otherwise, is not a consideration). Those sentenced to 12 months or less are released unconditionally. Those serving more than 12 months serve the remainder of their sentence "on licence". Furthermore, those serving sentences of between 3 months and four years are usually eligible for release under the "Home Detention Curfew" arrangements which sees many of them serving only one quarter of their sentence. Only if they commit further offences whilst in custody (for which they must be prosecuted and convicted by a court) can time be added to their sentence.
There is an exception and the Rolf Harris case possibly fits into this category. A Parole Board decision (where behaviour may be a consideration) is necessary on the release of prisoners sentenced for a serious violent or sexual crime committed before 4th April 2005 and for which they received a sentence of more than four years. (I'm not sure without looking it up whether Harris's offences fall into the "serious sexual offences" definition.
Prisoners used to get up to a third off their sentences for "good behaviour". Prison governors had powers to order a reduction in this remission for bad behaviour. Some years ago this was deemed unlawful as it was seen that the sentences were being arbitrarily increased by somebody who was not "a properly convened tribunal".
In January 2013 Chris Grayling, MP, the Justice Secretary, sought an end to these current arrangements which were introduced by the Labour Government. He announced plans to review the current release arrangements and return to linking early release with good behaviour. It's only 18 months since then so, unsurprisingly, nothing's happened thusfar.