If there's an ongoing fault with your broadband and Virgin Media have issued you with a reference number and a review date you may be able to leave at any time. Ring them in 150 from your landline and follow the options until you get to the 'thinking of leaving us option'.
As has already been stated. If there is a known issue in your area and a fix is not forthcoming then you have the right to leave mid contract without penalty. You will need to speak to the cancellations team in order to get this arranged.
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