'Define a group though. Individuals all staying 2 metres apart - are they a group? In which case individuals walking, two metres apart are a group then.'
I think the definition of group goes to organisation and prior knowledge of those people.
So, a yoga instructor and her class in the park...that's a group.
Five people who don't know each other who just happen to be in the park at the same time...not a group.
The difference is that in an organised group or with friends you are much more likely to break the social distancing rules. The temptation to drift nearer is too high.
Also, remember that 2 metres is a minimum not a target.
If you go to the park and see a stranger doing yoga, you're not going to go 2 metres away and start doing yoga. You're going to move away further and find an area with more space.
In a class scenario, you're leaving 2 metres as a maximum so you can all chat/see and hear the teacher.
I hope that helps.
This is the sort of thing where common sense comes into play. The fact that you can't see the difference means your common sense may be being affected at the moment, probably by the stress of the current situation.
As you say, you're staying in anyway, so it doesn't really matter that you don't understand groups vs individuals. Just hope that the people who are outside have a better understanding!