You might get a better offer from an outbound retention call after you've served notice of cancellation, but the comments in the forum suggest that VM are nothing like as persistent as they used to be in this respect, so banking on this is a risky strategy. Even if the call comes, if you're in a marketing segment that VM regard as "price tolerant" then they may still only offer a paltry discount. These days it seems that discounts also involve an 18 month fixed term contract (which is big bit crap given VM's exorbitant early termination charges) so factor that into your assessment of what to do, along with the fact that VM have "missed out" on this year's inflation busting annual price hike, so we can expect a big increase in 2021 (and your discount is an absolute amount, so you'll still get hit for VM's price hikes)
If you decide you're changing ISP, then there's some excellent smaller ISPs - my personal favourite is Zen Internet, but AAISP, Aquiss, uno, and IDnet all have similar or better customer ratings. If you go with shysters like Sky/Now or Talktalk, then you'll have a similarly poor offshore customer service experience, after the UK based sales team have locked you into a contract.
Whoever you think of signing up with, before you commit, check out a few pages of customer feedback on Trustpilot, and the company's overall score (and perhaps contrast that with Virgin Media's ratings as a benchmark).
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