Overselling.
It's a disease.
They don't give a toss about whether or not you get the speeds advertised, they care about you signing the contract.
And if Virgin have oversold their bandwidth then BT has probably done so several times over.
This is what you get when you a) live on an island and b) have a succession of supine, cretinous politicians who are so stupid that they don't know what questions to ask or what the answers mean when someone tells them what to ask.