There are lots of cases in Italy outside the lockdown towns, and I think it's extremely unlikely that every case has been caught by testing.
I think people are far more at risk of catching Coronavirus in Northern Italy than on the tube- it's the airports they go through as well as their final destination. There's no real evidence to suggest widespread community transmission in London (yet).
Also, the Italian health system is becoming overstretched.
I hope anyone going skiing would be able to cope with quarentine + self-isolation on return!