If he is found guilty, the chances of him going to prison is very high. One has to consider the potential effect his 'lie' has had on the UK.
Even if he is found not guilty, do you think he is guilty?
Simple test:
As a public officer, did the public expect him to know the true figure?
As a public officer, but not in a public officer role, did he knowingly use a higher than accurate figure?
I'm not sure you can take into account whether his 'lie' had any effect on the campaign.
I'm not sure you can take into account his motive for using an incorrect figure.
I personally think that the test, "was he in a public officer role or not" is an unfair test. The general public would not apply a "is he speaking as a public officer, or in a private individual capacity" test, when listening to him.