There is much to criticise Boris for since he became PM, but partygate? That's like being outraged at Fred West for failing to secure planning permission before digging holes.
Boris was likely telling the truth when he said he didn't realise he was breaking the rules, due to his sheer laziness of not knowing them. I'm betting he followed the example of his own advisers who were throwing unlawful office parties and figured he was in the clear for his birthday.
Rather than tutting in the Commons and expressing horseshit outrage about partygate, perhaps they ought to focus on the far more serious issues at hand affecting real people: the ridiculousness of the migrant situation for a start. But Labour won't do that, they would consider putting up 'Vote Labour' posters around Calais as 'planning ahead.'