They’re doing what the previous government, who brought in this daft law, and the current government, who supported it legally require them to do. They know it’s stupid, but it’s not Ofcom, it’s the politicians.

Spot on. The letter is a prime example of a boilerplate process that Ofcom have developed to meet the requirements of the law as it currently stands. By now I expect they have no illusions about 4Chan's likelihood of complying. I wonder how long they will carry on sending similar letters and giving Preston Byrne an opportunity to have another laugh.