It could also be some extremely heavy user going away on holiday for a week.
Considering a single channel on a UBR is around 40mbit, a few user(s) taking the "No Download Limits" to the extreme could have a pretty serious effect on several hundred, maybe thousands of customers connected to that UBR. The current design of the network really isn't meant for these high speed. The low bandwidth of a channel + a high amount of users don't really mix.
This is something that will be helped alot by the DOCSIS 3 rollout If/when that happens but we all know this will cost alot of money which is going to be difficult for VM to rack up.
In the mean time, they have to try and keep the current network balance by whatever methods they can on a somewhat limited budget. Same with alot of ISPs right now
luck of the draw I guess. On the bright side there is alot of talk about the FTTC at the moment.
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10Mbit Virgin Media (ex-BY)
4.5Mbit ZEN ADSL Lite
3.5Mbit ZEN ADSL Active