Check on Uswitch for what you'd pay with other ISPs, making any adjustment you see fit for the speed offered.
I don't wish to be critical, but for the benefit of anybody else reading this, it always pays to do your research before calling VM to negotiate a new deal. That's because (a) you know what the field looks like when you're discussing with the agent what they can do for you, and (b) retentions deals don't sit on the table whilst you go away to think about it. Just because you were offered £34 last time you called, the next time you speak to them there's no obligation on company or agent to offer you the same price. In many instances the agents will insist that deal wasn't available, or shouldn't have been offered, and sometimes that's true, other times it is probably because their performance bonuses are built up party by retaining customers, partly by not giving away discounts too easily.
Personally I'd have had their hand off for that deal, you now need to speak to them again and hope that the agent you speak to this time can and will offer you the same deal. I doubt you'll get it lower, but I can't say that as a fact. You could try cancelling and hoping for an outbound retentions call, but if you've refused £34 a month, then you're already way below VM's average revenue per user of £52 a month, and potentially they won't go lower, and might even conclude that a retentions call is a waste of time. And if you cancel, but have no intention of leaving, if the retention call doesn't come then you need to phone in to cancel the cancellation, and probably accept whatever they are willing to offer pricewise, as your leverage is suddenly reduced to zero.
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