I'm not saying that what he did was a good idea, but for those who claim that 'rules were broken', and so far his only defensive claim has been that 'no rules were broken' it may be worth considering what the rules were under 'Tier 2' (England):
Guardian
Or Gov.UK

The 'no mixing of households' only appears to be listed under 'meeting friends and family'
For 'work and business' it says only that "Those who can work from home should do so"
Anything else, 'keep 2 metres apart' 'wear a mask' etc. seems to come under guidance, not regulation.

I can almost imagine this running a couple more weeks and Boris then saying "Ok. what rules are we supposed to have broken?"