Yes, by the ASA. But I'm not sure it applied to simply speed-limiting, as opposed to virtually or completely stopping the service. 2Mbps is still a lot more than many get on xDSL. It seems this Three restriction, from its wording, may not be an automatic and inevitable speed limit anyway. It may be the pattern of usage is also monitored. We haven't seen the poster's warning text either.
Three have tried to cover themselves anyway in respect of the ASA, by specifying that such volumes would look like commercial work on a non-business contract.
From his post anyway it looks like he was just wanting to know what would happen if he cracked 1TB.
We don't know by how much he exceeded it either. He may have kept going even after the speed restriction. In fact it is clear that he did.
It's a shame he didn't just ignore or accept my comment about that being daft, instead of trying to justify it. I hope he won't regularly be doing this. He did say it was just to find out what happened.
I don't see much point in arguing the permissibility of it or not with the poster. I doubt if many will be hitting that figure regularly.