Surely all the dancers would be making sure that while they are still on the programme that they keep well away from everyone, it can't be that hard, don't they spend most of their time either training or filming the programme? To not have close contact with anyone else for the rest of the time can't be that hard can it?
They’re just people with a job to do. They still need shops, have families, all that stuff that normal people with normal jobs have and do.
Katya was asymptomatic and could have got it from anywhere in her working life or personal life. Just like any of us could, unless we’re able to shield completely- no kids at school, or partners at work, no one to bring it to us in our glorious isolation, for months and months.
Dancers on the programme are working. In order to work, they have to be in close contact with their partner (who can also be in close contact with others if they have a family, who might also ...), and go out and about every day to a studio (where other people work, who might be in close contact...) and be filmed every day (so extra people around, who might...) and sometimes be in 1 TV studio (It Takes Two) and sometimes in another (main Sat show) and have their hair and make up done and probably get a taxi or driver and so on and so forth.
With every precaution, you’re never going to eliminate risk entirely.
‘Bubbles’ aren’t some magical protection. They just reduce risk, not eliminate it.