As I said yesterday and Dominic Raab confirmed in the media this morning, the Government advice is that you can meet others as long as it's outdoors and is socially distanced (2m).
As he quite rightly put it, please 'use your common sense' - if you're outdoors and you can maintain social distancing then there's no harm in sitting on the next picnic blanket down from your sister in the park, but if there are reasons why you may not be able to maintain social distancing - such as perhaps young children from different households who may not understand or might get upset when they can't play together - then you should not do it... These are judgements for you to make.
If you feel the advice on this is vague, that is because Government is asking you to please exercise your good sense and judgement, and to make decisions about individual situations for yourselves. We cannot produce a manual that covers what to do or not do in every possible social situation.
The reason it's limited to outdoors is explained in the article below, but in short:
When outdoors, people are much less likely to come into contact with an infected surface, while any tiny particles of virus (called aerosols) would be dispersed by fresh air.
So the main remaining danger comes from large droplets, and staying 2m (6ft) away from other people - as social distancing guidelines recommend - should overcome that.