For a different take on the same question, there's always the editor's pick over at ISPReview. Or Ofcom's complaints report.
Fundamentally not much changes when you read those - big companies are all poor (although VM seem to be notably poorer than the average of large ISP), they are all high-marketing spend, economy customer support (at least BT is UK based), and use heavily discounted offers to attract new business. When any of these companies' service works (most of the time for most people) then there's no difficulty, and we rarely hear from customers whose service meets their expectations. When things go wrong, that's when you discover what customer support means, and it is almost entirely those customer interactions that are being graded in all of these things. Relatively few customers do their research and make a considered decision on all available data - instead they get swayed by fancy and repetitive marketing, and that is why the big players all spend £100m+ annually on marketing, use UK based sales call centres, and then (mostly) have crap, offshore-based dumpster-fire support, and how-can-we-drop-this-call-quickly inbound telephony.
If you want good service, you pay a bit more, and pick from one of the companies on page 2 of the ISPReview editor's pick. Even for those, there's complaints - the point is there's a lot more happy customers taking the trouble to review smaller ISPs than with the big companies. I've dealt with Zen on a number of occasions and would happily recommend them, but there's still complaints from some customers. It's worth reading the reviews on AAISP, in particular if service is more important than price, but note the smallest ISPs sometimes have different commercial offers, - a very few have charges for tiers of data use, some don't provide a router, at least one doesn't even (by default) price in a VDSL modem. Approach reviews like you would on Amazon - does the complaint reflect the reasonable responsibilities and performance of the company concerned? Are there trends of repeated themes? Insofar as it can be judged, does the reviewer appear to know what they're talking about? Does a review with poor ratings seem justified by the accompanying text? These filters apply as much to VM, TT, BT as to smaller company reviews.
In the case of Openreach VDSL, a supplier can't move a customer closer to a cabinet - so disappointing speeds for that customer can remain a fact, without being within the gift of the supplier to fix. In all other respects, just because Openreach ISPs all use the same local loop, it doesn't mean the service is the same. Many people think that it is the case, as it is with electricity, but with ISPs, there's a lot more variability than is seen - different backhaul, different traffic routing, different capacity decisions, different default DNS, as well as a vast difference in the willingness and ability of the ISP to get the best possible outcomes from Openreach.
Coming back to the question of do VM listen across different sources for feedback, the answer is an unequivocal yes. They will have a team who do a round up of all social media content and external newsflow for the company and for competitors. The company also have something we can't see, the extensive data from support and service calls, from cancellations discussions, and from their own customer surveys. Those will provide about 100x the volume of data visible in all public review sites. From this the company know full well what customer experience is like, and it seems an inescapable conclusion that they know current standards of service are often poor, but not sufficiently damaging to financial performance that they see a need to make a material change. There are things VM are improving - reducing non-fault call out costs, ending early disconnection fees for customers moving out of service areas, keeping in bundle roaming rates for mobile, the great Volt offer, there's been a modest increase in UK support staff, and the (long term) plan to convert the VM network to FTTP is very welcome. On the other hand first line support is still too slow and difficult to access, and often poor when customers get through, and DOCSIS remains an ageing and problem prone technology.
As always, VM is great when it works and you don't need to contact the company. My heart sinks at any time I need to contact Virgin Media. By comparison, any time I have to speak with Privilege Insurance (or other companies of the Direct Line Group), I think "great, my call is going to be quickly answered, by a friendly, competent UK agent who will quickly resolve the issue I'm phoning about". Comparing insurance to and ISP isn't the same thing, but in terms of business magnitude, average customer value, and customer contact structures they're very comparable. VM could offer better, UK based service, it would cost less in the long term due to faster call resolution, better retention, improved reputation, and fewer repeat calls. But the considerable pain, short term costs, and difficulty of transforming from sh*tehawk to swan is the barrier here, along with the mediocre US management who evidently believe all the crap spewed by management consultancies about "wage arbitrage" and the advantages of offshoring and outsourcing.