I don't have the 20mb package but I don't believe that VM do anything of the sort unless they have identified a customer as breaking their 'fair usage' policy. The problem in your case is far more likely to be the fact that they've got too many customers in your area and the contention ratio is now far too high. The fact that VM have dropped your bill to £25 and offered you a better TV package suggests to me that they, too, are aware of what they've done and are trying to keep you sweet.
Can you get a better deal with a telephone line based ISP? The only one that does both TV and BB is Sky so it might be worth asking the question but I doubt if Sky could offer you anywhere near 20mb.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.