From A&A to Z?
I use Zen myself, and would agree that their service is very good.
I’m a little surprised that there is no mention of the equally good A&A, or are they perhaps just a little too niche a provider to have been noticed by Which?
(I also wonder if there’s an element of bias in the results? More techie people are perhaps more likely to go for more more techie-minded ISPs, and so if something does go wrong we’re perhaps more likely to know how to fix it (if the problem is at our end), and so complain less? For example, my router tends to take a huff if left online for more than a couple of weeks at a time (my hunch is that its DNS cache (or something similar) gets filled up and it eventually gets harder for it to look up hosts. So I have it plugged into a timer switch in order to power cycle it once a week: problem solved (or bodged around, anyway))