According to the news on BBC it is only a legal requirement to self isolate if you are spoken to in person by someone from track and trace, the APP is only a recommendation.
The nhs scotland app, I enquired about, if I'm required to isolate for 14 days, if I get an alert, from the app. Have now been told its a legal requirement, and you will have to isolate for 14 days
But all of this will depend on how long you are exposed, and how near you were to the infected person. So for the nhs-scot, your required to be exposed for 15 mins, and distance wise 2 meter, if you meet these parameters, then you will be asked to isolate
If it was only guidance, and you went into work, and then a few days later, become ill with covid-19, there a big risk it will be past on to others