Perverting the course of justice is a crime.
The sentencing guidelines are here.
Pleading guilty will reduce the sentence. Having stoutly insisted upon innocence before that will increase it. Note that sentences are expected to be consecutive, not concurrent, and there should be two, one for the offence and one for the perversion.
Sentences are reduced/increased given the seriousness of the underlying crime.
The current betting from those who know more about this than I do is over a year of jail time (no, whatever it is will not be suspended) and thus a by-election.
Anyone want to lay claim to a different number?