The latter I'm afraid. Nominally we have 30 mbps fiber but the pocket loss is so bad usually that it's more like 1 or 2, and Flash just gives up loading videos. Often it gets so bad I just use the 3G on my phone instead. Also, there is a 5 gig quota in the morning and a 9 gig one in the evening, which basically means Steam games have to be downloaded in two batches.
4 of my friends are sharing 21 mbps O2 ADSL and it works fine, so we're definitely going for that next year.
Richard Branson's Virgin Media has got it more or less covered
As a Virgin Media customer in Kensington, I can say that's a load of horsesh