Anyway, I need to research VM alternatives before I properly engage but it makes me wonder if people are jumping ship in unusually high numbers.
In a word, yes.
Any sizeable price rise causes people to re-evaluate, and thus triggers a lot of losses (when customers have the option - next year's mongo price rise won't give customers that option due to the new VM T&Cs). With so many of the big players announcing huge price rises, if all else is equal, that is a bit of a zero-sum game where existing customers churn out, but new ones join on the very highly discounted new customer offers, and it takes a few months to see who's won and who's lost. BT changed their T&Cs last year, so most of their customers don't get the option to bail this year, and that means VM customers can bail, BT can't, and so VM are at risk of a net loss of customers, as well as the unfavourable effects of new customer pricing.
Last year VM put broadband prices up by 5.5%, and actual revenue per customer was down a couple of quarters later, so clearly VM lost then (and that before the BT terms changed). But the battle isn't just about the offered price, it's about marketing - most people overlook the smaller ISPs who'll offer fixed price deals and twelve month terms, and instead go with the big names that endless TV adverts have etched onto their retinas. Plenty of other ISP's customers are sitting on their couch, watching TV over an extra large pizza, thinking "my non-VM ISP is putting up prices at the end of my contract, I'll go to VM, as I've seen their adverts offering Squigabit internet, 1,000 channels of tripe, ten O2 sim cards, best-ever wifi, and a landline with unlimited calls for thirty quid".
So long as people agree to contracts with ludicrous baked in price increase schemes, the big players will ream them out. In return many customers zig-zag between large ISPs, tolerating the crap service in return for massive up front discounts, now with an inflationary sting in the tail. Those less savvy, those whose lives are too full to frat around switching suppliers, and the vulnerable, they'll just keep paying full price and be each ISP's most profitable customers.
My price increase this year? Nil, because I'm with a smaller ISP.