In the case of copyrighted material the letters are just to discourage occasional users. The ISPs send out four warning letters then stop sending them without further action.unless something has changed an internet provider & can see how much bandwidth a client has used & when it was used but they are not permitted to look any deeper into what you were doing with that bandwidth be it watching &/or downloading as that would be violating the data protection act... correct me if I'm wrong!
They can suspect what your doing & send out scary letters but that should be all that they can do without specific permissions being granted.
In the case of live streaming or "super blocking" the waters are muddy. What does this term mean; "The latest technology advances that enable the ‘super-blocking’ model do not allow for the identification of end-users, the Premier League said at the time."
To the ISPs that could mean 'we don't need to know who is streaming the sport for us the restrict the broadband'
Premier League renews super-blocking order: reports