[pmyteh]

There is (for most crimes) an entitlement to release on licence after 1/2 or 2/3 of the sentence, with the rest being served in the community but subject to recall to prison. So the most important takeaway is that the 'headline' numbers are higher than those actually served. Given the crime and the date of sentence it's very likely that he served three years and eight months inside. Some of that might have happened before his trial if he was remanded in custody: time spent in prison pre-trial is counted towards the sentence.

The 'additional' time for not paying £963k is designed to be coercive. The court has, in theory, decided that not only does he owe the money but he can be expected to find it. And the prison in default is supposed to encourage him to do so. Importantly, it's not an either/or: he still owes the money even if he serves the default sentence.