These "didn't get the deal I agreed" situations happen from time to time. 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 (Consumer Rights Act 2015), so VM have to honour whatever was agreed, whether they like it or not. There are no "get out of jail" clauses for VM, such as the agent got it wrong, deal not available to you or other nonsense.
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, not some less attractive compromise. 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 don't pick this up and get it sorted in that complete manner, you need to search, read and follow the Virgin Media Complaints Code of Practice.
If you do need to 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 request they issue a deadlock letter to allow you to immediately escalate to the industry arbitration scheme CISAS. In the meanwhile, don't do anything daft like stopping your direct debit, that'll be recorded as a credit default, and opens up whole new realms of difficulty for you. Don't worry if VM contact you claiming that they couldn't get hold of you, so will close the complaint if they don't hear to the contrary, this doesn't stop you being able to take the matter further.
If VM still won't honour the contract despite the complaint, or say they're going to close the complaint, then you follow the process and escalate to the industry arbitration scheme. If they haven't issued a deadlock letter then you will need to wait eight weeks after your initial complaint was received by VM, otherwise CISAS won't be able to accept the complaint. 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. Not only does escalation to CISAS cost VM money (free to you, costs are ALWAYS paid by the company) but it gets your case investigated and impartially heard by expert dispute resolution staff. Sadly, if it goes that far it is not quick, on the other hand your legal rights are very clear.
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