That should have been :
My Union (Unison) has advised we should ask our employer (large public organisation) about any steps to take with regard to colleagues travelling to and from northern Italy
Nothing the union can or will do, and they advise against taking any action without management agreement.
The union support the employer position, and say all the employer can do is to follow PHE advice, which is to self isolate if you've been to one of 10 Italian towns.
If employees choose to leave work, including work from home, because the person sat next to them just returned from Northern Italy, they will be seen as at fault Not the colleague who returned from Italy.