I'm not sure this is going to help. This is mine and I would hazard a guess that the Brit version is the same GI Joe stock piece with different stenciling.
The string (Rope!!!) seems fairly standard and must be easily replaceable from a good quality string shop!!!
Stringy is roughly 63.5cm long or 25 inches for us painted heads but looking at mine I've no doubt it's shrunk a bit with wear and tear etc over the years because it's been well loved and well played with... and that's the way I like it, baby, I don't wanna live forever!!!!