It is dumb for virgin media to not hash the passwords, however them mailing the password by mail is a completely normal procedure;Banks literally mail your debit or credit card.
I am not going to go digging into country specific and international law to see whether the act of not hashing passwords is legal or not, however I think it is idiotic to think if it truly was illegal, their legal team just missed it or glanced over it.
As for Deadmonstor, what you linked literally doesn't say what you think it says, the link that you give is a "password guidance approach", it is not an actual law there are no statues for the so called law, there is no penal code, there is no paragraph number, you are simply miss interpreting their "SHOULD".
I SHOULD WIPE MY ASS AFTER TAKING A SHIT BUT I AM NOT PUNISHABLE BY LAW IF I DONT
On the topic of hashing, most php based databases use md5 hash anyways which takes mere seconds to crack.
I also love that some people are acting like this little thing is actually going to tank virgin media, if their company would be public we wouldn't even see their stock decline by pennies under a magnifier.