This isn't a new issue, and it predates the internet. There were publishers of magazines containing pornography (or anything else unsuitable for children). These were sold in shops. A publisher had to ensure that the material in the magazines was legal to print, but it wasn't their responsibility to prevent children from looking at their magazines, and it's difficult to see how that would even be possible. That was the responsibility of the people working in the shops: they had to put the magazines on the top shelf, and weren't allowed to sell them to children.

On the internet, people don't get porn videos directly from pornographic web sites, just as in the past they didn't buy porn directly from the publishers. The videos are split up into packets, and transmitted through an ad hoc chain of servers until it arrives, via their ISP, on their computer. The web sites are the equivalent of the publishers, and ISPs are the equivalent of the shops. So it would make a lot more sense to apply controls at the ISPs. And British ISPs are within the UK's jurisdiction.

And before anyone points out that there are workarounds that children could use to bypass controls, this was also the case with printed magazines.