In principle yes, it would work. But because higher speeds don't involve any additional cost to VM, you're better off focusing on the technique for getting a discount than "going slower". And to get a worthwhile discount you need to know your stuff:
(1) Know what the alternative offers for other ISPs are and have them in front of you when calling, and remember that any price difference per month is multiplied by the 18 months of the contract. This is potentially a £700-1,200 contract you're negotiating, treat it with the same energy and attention that you would for any other purchase of a grand.
(2) The way the retention agents work, you may get different offers on different days from the same agent, and different offers on the same day from different agents. If you don't feel you're getting very far, end the call and try again another day - although the next agent will know you're a repeat caller, and will have to be pushed harder as they'll assume you're more inclined to stay than leave. Sometimes when you call them, they won't offer decent deals, and then you need to consider cancelling and hoping for an outbound retention call during your notice period (see item 6 below).
(3) Learn the craft of "retentions discussions as a customer". In all of this, your leverage comes only from the extent to which the retention agent believes that you are genuinely prepared to leave, but can be retained for a discount. These agents do this day in, day out, they're trained, they're usually experienced, they're paid on results, and it's pretty common to up the pressure on them by having league tables up in the office that show all agents' performance. Every caller says they want a discount or they'll leave, many customers aren't prepared to cancel, and the agents are good at sniffing out those who show any of the signs of not really intending to leave. Some customers can be distracted by an add-on offer that wasn't something the customer requested, but seems like a good deal, or by some sales patois about the Connect app, public hotspots, "best ever wifi" and the rest. Focus on what you want, what you can get elsewhere, and have in your mind what is required to convince you to stay with VM - maybe write these down. If this is about price, avoid starting any prattle about loyalty or fairness as that language instantly tells the agent you're likely to stay. Be wary of retention hooks like bundled sims or TV packages they start offering unless you really get some value from those. Many people get distracted by the offer a free wifi pod and a Volt bundle that they wouldn't otherwise have chosen, and because they think they're getting something great, end up paying £7-20 a month more, which adds up to quite a tidy sum over 18 months. If that's actually what you want then by all means go for it, but always have in mind what you want from the call.
(4) Regardless of what the agent might say, any offer completely lapses if you end the call "to think about it". Know what a good price is before having the conversation, and if you're offered a great deal, then accept it immediately, equally don't feel pressured to accept a poor deal compared to other ISP offers.
(5) Keep it brisk, polite and chirpy - these guys and girls are doing the job they're paid to do, getting a mutually agreeable outcome quickly and with a friendly discussion is ideal for them, ideal for you. Time is money to these people, so start off the call by giving the agent a good idea of where you want to end up - if you've caught their name when answering the phone, make it personable, eg "Hi Dave, I'm hoping you can help me as I'm at the end of my fixed term contract (or "had a huge price increase letter"), and I think I'm going to have to cancel as the new contract price is so far different from the deals I can get from (eg) Plusnet/Sky, for £xx a month. Can you match those prices for me?". They'll usually try and offer something above those other company offers, if so come back along the lines "Thanks Dave that's in the right direction, but it's still well above what I'd pay elsewhere, and that £x a month difference is going to be £xxx over an 18 month contract - is there any more leeway to match the deals easily available? I can commit now if you can match that price?" If trying to deal with any offshore agents, then your experience is likely to be materially worse than the UK based retentions agents, although regardless of UK or offshore they're all outsourced staff who actually work for companies other than VM.
(6) If you really don't want to bother with the retentions pantomime, and just issue notice to cancel, then there's a good chance that you'll get one or several "outbound retentions" or win-back calls during the notice period. Make sure that in My Virgin Media you've ticked the box to permit VM to contact you for marketing purposes! These callers have access to the best pricing and are more likely to offer you something around new customer pricing than an inbound call you make to the retentions team. Obviously there's no guarantee that these calls will be made, or that you'll be available to take them, so they can be part of your renewal strategy, but usually in conjunction with placing an order for a different ISP. If the VM call comes and you agree terms, you cancel the different ISP under cooling off rights, and pay nothing. If the VM outbound call doesn't come, or the terms remain unacceptable, then you allow the cancellation to run its course and use the new ISP connection.
(7) Be aware that under the new T&Cs if you stay with VM you'll be hit by a price increase next April, and under the new T&Cs you won't be able to cancel or renegotiate. Whilst next year's price rise will be RPI + 3.9%, that's on the undiscounted price, so what you'll see is that it is magnified by the proportionate discount you have, so the better the deal you agree now, the higher the monthly increase you'll see then. Maths of this is explained in this post. but as a budget guide assume that next April's price rise will be around 17-22% against your discounted price. If you can get the agent to agree to a "fixed price for 18 months" deal rather than the more common fixed discount arrangements, then that's legally binding, but chances are you'll need to use a DSAR to get the call recordings in order to enforce your rights should VM insist that you didn't agree a fixed price. Agents are under pressure not to offer fixed prices, so might agree verbally (still a binding contract) but not put that on the system.
If you decide this is all too much and that you are changing ISP, check what you're signing up for - other big ISPs are generally doing the same tactics with unavoidable and unfair price increases, but Zen Internet are worth a look and offer 18 month deals with a guarantee of no price rises in the fixed term. Talktalk or Vodafone and the other big ISPs will offer really attractive up front prices, better than Zen, but they have adopted similar terms to VM, so you'll get stung when they spring the non-negotiable price increase next year. Personally, I bailed out earlier this year, I'm with a smaller ISP on a 12 month contract with no in-contract price rises, and whilst headline price is higher than VM's discounted offers, this supplier doesn't use huge discount to lure customers in, so I won't have a massive price rise when my fixed term ends. I've had enough of poor service and the misery of playing the retentions game, but some people enjoy it.
