Stick at it. These "didn't get the deal I agreed" situations happen from time to time - in fact they happen regularly with Virgin Media. However, your legal rights are crystal clear - information provided by a company verbally or written is binding if the consumer relies upon it when entering a contract (Section 50 of the Consumer Rights Act 2015), so VM have to honour whatever was promised, whether they like it or not. No matter what VM say about "not the right deal", "can't offer that to you", "deal not on the system", you agreed a contract with their agent, that is now binding . And perhaps they need to re-train some of their staff because yours is a regular complaint in these forums.
Hopefully the forum staff can pick this up and get it sorted - by which I mean that you get exactly the deal you were promised, suitably backdated to when it should have been effective, and an appropriate goodwill gesture (say £70) for the inconvenience and stress. That would be the cheapest and quickest resolution for both parties, and the best way of trying to recover some goodwill in a bad situation. If they can't get it sorted in that complete manner (and don't settle for miserly goodwill payment), you need read the Virgin Media Complaints Code of Practice and raise a formal complaint. I suggest in writing, by recorded post. Reject the contract they're trying to impose, and demanding that as per your legal rights the company honour what was promised, and pointing out that if the company can't honour its agreement you will escalate to Ombudsman Services as soon as permitted by the rules.
If VM still won't honour the contract despite the complaint, or issue a fob-off resolution that doesn't settle things acceptably, then you reply by rejecting the "resolution" and asking for a deadlock letter, with that you take the matter to the Ombudsman. If VM stall and you can't get a deadlock letter then (ignoring that it is a breach of regulations) then eight weeks after your initial complaint was received by VM you can take the matter to the Ombudsman without a deadlock letter. Explain the background and request that VM be required to give you the promised deal, fully backdated, and add on a request for compensation for the hassle.
And apart from the first three words, all the above is boilerplate text that I have saved because I need to use it so often. Usually the forum staff will sort this out for you without needing to go to complaints and the Ombudsman, but that route remains open if they can't sort it, or won't offer a reasonable settlement offer.