Slightly off topic, but I was with Three UK for a while. Reception was universally terrible, and trying to use mobile data was a joke. This was in a small urban centre... called London. Would never, ever touch them again. Back on topic, it seems that they are not using Bluecoat/Symantec Site review (which I suspect other providers do), which has the domain categorised correctly: https://sitereview.bluecoat.com/#/look…
Over the last 10 years I've used Three a few times for a short while but their reception where I am (in London) has always been awful. However, most recently I tried their 5G home broadband. It worked pretty well but I found a much better deal with Virgin Media so I tried to cancel under their 30 day "money back guarantee" and I've never had such a headache. Their store staff repeatedly lied to my face that it didn't…
Your best course of action is to never sign up to a contract (use the 30-day rolling plans) and be ready to switch at the slightest issue. For home/fixed-line, consider having 2 providers for redundancy so you don't lose connectivity while switching.