Yes you can go outside for exercise.
If you read the screenshot chestnut you can see that it IS talking about people with symptoms!
It is bollocks that you can be fined or jailed for leaving the house within 14 days of a family member showing symptoms.
For a start, there is no register of people self isolating so will the police know which jogger to arrest?
Quarantining the whole family after ONE member shows one mild symptom is the right thing to do, but it is a hard thing to do.
Many won't, and will keep the cough their child had last week a secret.
So I am far less worried by the sensible self isolators taking their child out for a socially distant walk than I am about those who have kept their symptoms secret who are still going to the shops etc.
That said, it's so widely misunderstood that I have not left my home at all while self isolating (I don't want other people to think I am breaking the rules, especially as our neighbours have been so kind in dropping off food etc).
But we have a garden. If I didn't, I would take the DC out for fresh air and socially distant exercise, and I would be allowed to.