Once you've agreed to the terms and conditions for any online renewals or regrades made through My Virgin Media or via an email offer that was sent to you, the orders go to a back office team who apply the deal. This can take up to 14 days to process and they'll send you an email confirming the change has been made.
Changes made over the phone are usually actioned immediately (unless the change also involves new equipment, in which case that won't complete until you've received and activated the new equipment)
You could contact the Sales or Customer Services team over the phone (150 from a VM landline or 0345 454 1111 from any other phone and ask them if they can process the renewal, however if this offer price is an online exclusive, then the Sales or Customer Services team won't have visibility of it and will probably be unable to match that price point.
(Once it's been 14 days, if the new contract hasn't been put in place, pop back and update this thread and a member of the forum team will be able check things further for you)