Right, this does make more sense if you under stand the new rules are all about the effectiveness of track and trace.
The new laws stress this and priortise formal settings which have clearly laid out rules and an ability / obligation to honestly engage and assist with track and trace.
If you are informally meeting your mates and family there isnt this same sense of responsibility and there isnt the same professional responsibility / company liability to report health and safety incidents. Its too easy for family or friends to have a drink and go 'oh fuck it' and then start hugging / kissing etc because there is no one with a clear cut 'official' enforcement role.
So yes it is about both priority making (hence why health and education are being promoted as reasons for exemptions) and why bars / restaurants are being allowed to continue to operate (not just for financial reason but also because prohibition leads to demand being met by things like illegal house parties or raves and clandestine restaurants, hair dressers etc - where it become much more difficult to track and trace).
People also are more likely to forget informal meetings that arent regularly scheduled and the names of everyone at a larger gatherings. I believe 6 is the optimum number that people easily remember.
The overall point is that social interactions have to be more limited, ranked in terms of priorities and controlled and managed in a way that makes it as easy as possible for fast and efficient track and trace.
At the heart of the increased restrictions is the weaknesses in the UK's track and trace system which the government is struggling to get a grip on. These problems stem from reliance on telephone tracing rather than on the ground tracing (which is why several hard hit councils set up their own local systems), lack of trust in government (not helped by discrimination and economic marginalisation and language and cultural barriers which the government have been slow to recognise and try and resolve at community level - because they operate in such a centralised and one size fits all mentality.
If you under the 'fuck it' principle, you understand its about track and trace being ineffective and you understand that behavioural modelling will probably suggest if you say '6 people' a sizeable number of the public will interpret that as 'oh well 8 is just fine then' (when its not) the new rules do make a lot more sense.