Well, " Misconduct in Public Office" is certainly an offence occasioned when:
• a public officer acting as such
• wilfully neglects to perform his duty and/or wilfully misconducts himself
• to such a degree as to amount to an abuse of the public’s trust in the office holder
• without reasonable excuse or justification.
I’m not sure his performance gets as far as “neglects to perform his duty” - even though he got pretty bloody close in some aspects of his time at the FO so I assume they would want to go with wilful misconduct using the false claims regarding the £350 million.
I’m not sure you could actually demonstrate that and even if you could show that his behaviour met the bar for wilful misconduct I think you’d trip up at “to such a degree as to amount to an abuse of the public’s trust in the office holder” because I think it would be extremely hard to show that his claims during the campaign amounted to an abuse of trust in either of his official posts - Mayor of London (from which he stepped down in 2016 anyway) and MP for for Uxbridge and South Ruislip - because they were in a clearly separate sphere.
I’ll be interested to see how it pans out but, honestly, I don’t think this one has legs.