I am not sure whether you Brits have had a similar love/hate relationship that we have had with AT&T here in the US, but I will say that until divestiture in the early 1980s, AT&T had delivered excellent products and services in general, with pricey long-distance service being the biggest complaint that ordinary customers felt.
Generally, until the Reagan era, we loved "the telephone company" and hated "the cable TV company" but now they are equally bad.
Luckily, I have lived in the southeastern US for most of my life, so I benefited from being a Southern Bell customer, generally regarded as being the best, or one of the best, of the "Baby Bells"
When AT&T re-assembled itself a decade ago, everything was changed and rather than the "benevolent dictator" that it had been for decades, it is now a horrible evil monster that exemplifies all that is the very worst in a utility company. I hate the "new AT&T" even more than I loved the old one.
So, I now go with Comcast, an equally evil monster company, but at least they deliver a solid fast internet connection, which AT&T consistently fails to do. I can't understand how they allowed the same connection, which had been good under BellSouth, to deteriorate so far, so fast.