"If you read exactly what Virgin Media say, it is not a fixed threshold that you have to cross."
I'd love to know where you read that.
"Broadband Size: L
During peak times, the top 5% on the Size: L package download at least 750MB of traffic each.
Any users hitting this amount during peak times (4pm till midnight) will have their broadband speed temporarily traffic managed – their download speed will be set to 2Mb, with their upload speed set to 192Kb. This will last for 4 hours from when the traffic management policy is applied."
With, later: "The way that our customers use their broadband is changing all the time, so it's possible that in the future we may increase or decrease the thresholds – don't worry though, we'll always let you know on our website if we're going to make any changes, and we don't expect the thresholds to change very often."
So expect to see further limitations announced on their website in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying 'Beware of the leopard' soon.