[daveoc64]

All of the major ISPs and mobile networks do.

The UK government threatened to make it a legal requirement that all internet connections have filters for adult content on by default.

The major ISPs and mobile networks voluntarily put filtering in place by default to avoid the regulation.

The filters can usually be removed quite easily.

While there are some smaller ISPs that don't put filtering in place, they account for something like < 10% of the market.