Lowest prices for M250?

That's about as good as it gets.  You might not actually be offered that price again as prices can vary from day to day and from agent to agent.

I don't, and haven't worked for VM, but have worked within other call centres as a retentions agent, and would suggest that all companies follow a similar format in that individual agents are given "save targets" and can offer different deals than those offered by other agents on the same day.

I know it sounds odd, but agents are given a set amount of allowances to offer at varying price points.

If they have used up their quota of lowest prices for the day or week, then they couldn't offer say the Mega Volt bundle for £100, but if the next agent hadn't reached their quota then they would be able to offer that price point.  It's also possible that one of the retention teams may be "trialling" a price point not available to other teams.

To be honest if you are fairly happy with a  quoted price whilst on the phone with the agent (or need to confirm it first with say your partner) then accept it as it probably won't be available again.  You can then take advantage of the 14 day distance selling window should you change your mind and wish to reject that offer. You would then go back to your original pricing, even if it was the out of contract price,  and package (or nearest equivalent) and then have the option of either cancelling or renegotiating.

You might get a better offer from the winback team if you do place your 30 days cancellation notice, but there's no guarantee that they will call you back.  Also you need to log into My Virgin Media and check your marketing preferences allows VM to contact you, as if it's turned off they wouldn't be able to call you.