I'm also looking into long drop hanging, with decapitation being desired if anything due to the risk of otherwise surviving or being resuscitated if the fall doesn't work out. Reading
https://www.capitalpunishmentuk.org/hanging2.html I get the impression the maximum height used in the 1913 table hangings is 8' 6", plus an additional 9 inches from 1939 onward. Which I would guess is 2.82m in total. So I'd presume that 6m would be enough. But I'm guessing it's probably also dependent on the elasticity and strength of the rope, position of the knot or eyelet, type of knot or eyelet, fall trajectory, whether the rope has been left stretched over a large period of time beforehand, "elasticity" of the anchor or how much the rope can move along it under strain, whether the knot has been tightened beforehand, your position relative to the "anchor" above you, and more.
It's unfortunate there aren't any statistics on lethality or decapitation probability published as far as I know, though I'd love to be proven wrong.
Either way, if you find any information or figure anything out, I'd be grateful if you shared it.