Any breaking of the rules in number 10 were not like other people breaking the rules. Any mixing there was amongst people who were already in contact with each other for work purposes, while two separate households meeting up, against the rules, were raising the risk of transmission.
A simple rule was needed, but, with care, rules could be broken with no real increase of risk - our houshold was isolating as my wife is clinically vulnerable, while my parents were isolating due to their vulnerability due to age. I did occasionally break the rules and visit them - but there was no real risk in two isolating household meeting up. It would have been very different if I had not been able to work from home and had to go into work - in which case, I would not have risked going to my parents and would have had to do my best to keep away from my wife!