The London tabloids are known for playing pretty fast and loose with the facts, so you never want to rely on them for a factual story. I think Clarke has been suing them, and, IIRC, he won.
Also, as a purely logistical matter, Clarke has been effectively crippled by post-polio syndrome for the last thirty years, at least. The man can’t even stand up without help, so these claims are ludicrous on their face.
I believe the tabloid that made the claim was doing it more to discredit Prince Charles (who was going to present the knighthood) than Clarke.