I was told there was nothing they could offer me at all. This seems odd no?
To be honest, not really, no. These, 'offers' tend to be generated on-the-fly and are based on your current package, and, critically, an estimate of what a machine algorithm 'thinks' you will go for and the probability of you leaving, this is all, naturally geared to maximise predictable income for VM.
And that's all fine, like I have said on here many, many times, VM owe you absolutely nothing, and similarly you owe them exactly the same - other than what your current legally enforceable contract insists on. In theory, there is nothing* preventing VM from offering your neighbour a package for 20p per month and offering you the same thing for £300 a month! You don't have to pay that amount and similarly they don't have to provide a service to you if they don't want to!
Now the good thing is that you have had fair warning that the prices are going up by a substantial amount, what VM hope is that you do nothing and just 'suck up' the increase, if not that, then you wait until the last minute, then call up and get a 'deal', yes you pay a bit less but are tied into a new minimum 18 month contract. What they don't want is that you use this time to investigate alternative suppliers, think very carefully about what really do you want service-wise, and when the increase comes, you call up and cancel - which is your right - but you are pre-armed with what the competitors can offer so are in a better negotiating position.
* yes, OK there are a few reasons why they couldn't, 'protected characteristics' would count.