As a US resident I'm having trouble following this article, possibly because situations are so different?
Here in the US, if you can't get high speed internet, it's usually because there is no service provider with a fat pipe running near your house. Creating a new building regulation saying a builder has to provide that connection would just mean there are large parts of the country that would become unbuildable. A home builder can not miraculously hook you up to a network when the closest physical connection to it may be dozens of miles away.
Conversely, if there is a fat pipe, you can probably get service even if say your older home has no wiring at all. Adding that wiring is typically included in your initial installation, which itself is often free as part of a promotion.
Anyway, I'm having trouble wrapping my head around a law that makes it a builder's responsibility to provide a high speed connection when I don't see how that can reasonably be done if there's no network to connect to.
I'll explain why building Ethernet into a house is a good idea.
Let's start with my house. When we got broadband internet over 20 years ago, I paid an electrician to run Ethernet cable to three upstairs rooms in my house. We got a deal, because my son, avid to have internet and with no decent wifi router available at the time, crawled around our unfinished attic to help run the wires. From each upstairs room, the cable ran up to the attic crawl space, across the attic to a central column where all the electrical wires ran. Thence, down the column to the basement, across the basement ceiling and up to my office where the cable modem was. Bingo, we had internet in four places for four people. That was an EASY job! I wish that I had had Ethernet run to the kitchen, the TV room, and the enclosed back porch. Even with 802.11ax wifi, wired Ethernet is faster and more reliable.
Try to add Ethernet to any house today, and the job would be more difficult, depending on the house construction.
The point of the UK law is, first, the government recognizes the importance of broadband service for its citizens in a wired and electronic economy, second, that wired Ethernet is way more reliable than wifi, and, third, adding Cat 6 cable to a house WHEN UNDER CONSTRUCTION is an almost trivial job. The incremental cost of running Ethernet during home construction would be a tiny fraction of the sticker price of the finished home. This is consumer-favorable legislation like we do not have any more in these here United States. Lobbyists for the home builders would crush a similar attempted law in this country, dumping dollars in the laps of the members of Congress, all of them, because the builders are clueless and do not want to change their ways. Federal and state legislatures are equally clueless for the most part.