Nominet is behind this
Given some of the comments below, it looks like this is some sort of "initiative" driven by Nominet to promote their latest fad TLD, since a large number of supposedly unrelated registrars all seem to be using uncannily similar spiels.
IMO these registrars are certainly in violation of the Unsolicited Goods and Services Act, the Data Protection Act, and probably more besides, but as the instigator of this criminal activity Nominet should also be held accountable.
Therein lies a fairly large problem, though, as Nominet seems to be a law unto itself, with basically no oversight whatsoever, much like the EPO.
Of course, referring to Nominet as though it's somehow a separate entity from the registrars it's supposed to regulate, belies the truth that it is in fact "controlled by a very small number of large internet registrars". So going after Nominet actually requires chasing the registrars that control it. And where does one go to complain about those registrars? You guessed it ... Nominet.
That's a nice little racket they have there.
Might be time to have a quick word with the House of Lords Select Committee on Communications, and see if they can put this dog back on a leash.