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There are unfortunately many conplications with installations to flats. In particular there need to be the legal rights (easements) granted by the building freeholder to allow the cable companies to install their wiring around and through the common areas. If the block was previously wired, that probably has been overcome, with Virgin Media inheriting the old C&W agreements. However it's always possible that those agreements were cancelled.
One then has to look at the specifics of your flat. There may well have been cables routed to you, but for some reason they were deactivated. It could be the old cables had a fault. There may be a localised lack of capacity. Only VM can know that.
You will need to persuade sales to get someone to do a site survey to identify what they issues are and why you can;t have cable when others in the block can.
There is nothing to compel Virgin Media to install service to you. They are not a statutory supplier.