Having the same issue myself at the moment. I spoke to Virgin and they said there are "short term" problems in my area they are looking in to. You should be able to check if you are seeing the same thing by logging into your my Virgin Media account, and checking for problem with your broadband. Not much help but at least you can keep an eye on it.
Ignoring the speeds from the Store, what results do you get using the Test Network and Connections in the settings app?
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