They never make good offers when a customer is still in contract. Once the 30 day notice has started they might do better if you phone them then, but the best of all offers are made by the outbound retentions team (ie they phone you). Unfortunately, whilst outbound retention calls during the notice period are common practice, there's no guarantee they will try, or that you'll be around to take the call. There's also the peculiarity of how retention agents generally get incentivised - a near minimum wage salary, with big bonus potential for hitting customer retention targets (eg see here), then for a set time period they'll get a fixed amount of discount to give away, and that set amount can itself vary, but how the agent uses it is up to them. That means different offers on different days, and on the same day from different agents in response to a similar customer situation. If you're forearmed with the best competitor deals, and tell the agent early on that you're not paying VM more than you would a competitor, they'll know you're not interested in small talk about what a fab offer VM is, and you'll quickly find out if they're going to price match or not. What you have no knowledge of is how much of the agent's retention budget is left and for what period. If they've had to dole out generously already, then they're less likely to cut you a good deal, but that's the way of things. And don't forget if you are negotiating, then no matter what the agent tells you, any deal offered is take it or leave it at that moment - ring off to think about it, and that offer disappears and you'd start again from scratch.
Personally, if I had an Openreach FTTP option I'd dump VM regardless of what offers they had, because Openreach FTTP is modern technology, unlike the ageing and fault prone DOCSIS tech that VM use: But your needs and preferences may differ from mine.
