Before things were codified after WWII in 'Judges Rules and Sentencing Guidelines', 'General Regulations for the Prison Service' and other Home Office regulations, it was very much up to the individual Governor of the Prison concerned.
Six months is a short time for a prisoner to demonstrate that they are a 'model prisoner' and any time off would reduce that demonstration time.
The prisoner in question was sentenced three times for three crimes at least - so not an innocent who fell in with bad company, but rather someone who was making a living by crime.
For no other reason than the three sentences, I believe that he would have served his full sentence.
Regards
Chas