Strange that some members of the public are confused as to what they should do.
The Governments directive is very clear. Stay at home. It's been saying this for a while now and it hasn't changed. Do not leave your house.
There are a number of reasons where the govt will permit us to leave your house and, if legitimate, you will not be stopped or fined.
But the mistake a lot of people are making is that they think that The Govt is encouraging us to go to the food shops. They aren't. Go when you are running out of food if you can't order it online. It doesn't mean you can go shopping every other day for something to do and a change of scenery.
It doesn't mean you should take up walking and go everyday instead of just on Boxing Day. The amount of people walking past my house has gone up 50 fold. They are like s flock of sheep who have no common sense. Stay at home, exercise at home.

As for construction works, the construction companies are making the decision for the nation by closing sites down, HS2 has stopped as has Taylor Wimpey and Persimmon. Leading by example.
Many of the Builders Merchants are only honouring current pre paid orders and then they will be closing. Any Builder who decides to plough on will run out of materials very soon.

Don't complicate the issue If you're in doubt. It's very simple. Stay at home