Sorry, I don't see the fuss here.

It's widely known that UK ISPs and mobile networks put filtering in place by default. They did this to avoid the government passing a law to make it mandatory.

This often blocks VPNs and sites that may or may not be used to access adult content.

There is always a way to remove the block, typically with credit card/ID verification of some sort.

Three might be blocking some things that other networks aren't, but I imagine you'll find the other networks block some things that Three doesn't.