So this was real, and as others have said, it was a compromise at the registrar, not at The Register. The probable reason why I did not see it: the co.uk parent domain changes had been reversed before I checked.

This incident is a good example of why I recommend NOT using one’s ISP nameservers. The benefit of cache hits being slightly faster is offset by the TTL being, on average, half of what you would get by doing your own recursion to the authoritative NS. And then you get to share in the cache poisoning with all the other users.

Yes, my recursive resolver could get a poisoned cache as well, but the window of opportunity for mischief is much smaller. And I can flush the cache if I suspect something is amiss.