While breast milk isn’t exactly dangerous contraband, as with other liquids, you’re only allowed to board a plane (or enter the country?) with a limited amount -- unless you're traveling with your baby. In which case, you can bring as much as you want, 'cause you know, that's exactly when you need it -- when your breast is available and open for business.
In this case, the rules at Heathrow Airport require that all liquids in a carry-on bag are under 100ml (or 3.4oz); unfortunately, Milano was carrying more than double that amount.
Heathrow explained more on Twitter: