Dido Harding acknowledges that 10s of thousands of infected people are not self-isolating, but then says her greater concern is people not getting testing.
But that misses the point that the reason many people (especially the poor) are not getting tested is because...

they fear having to self-isolate if found positive. Equally, evidence shows that they (again, especially if poor) don't provide contacts because they don't want their mates to self-isolate if they can't afford to.

In the end, all aspects of the testing system - getting tested, giving contacts, self-isolating - are undermined by the failure to support self isolation and the fact that, of the one eighth of the population eligible for the paltry support on offer, only 30% actually get it.

So Dido, don't try and separate out the numbers testing from the numbers self-isolating. In the end it all comes down to the same problem: the failure to support people to do the things required of them and required for the community.