Michael, no problem to butt in -- that's the whole point of a forum! If tangents weren't allowed, we'd all be stuck in the same circle (get it?)
I'd be mildly surprised if there isn't liquid tight conduit in England -- there must be some equivalent. In any case, the stuff that we know works as a cambium-saver/rope-sleeve has a spirally wound steel core that provides the required structure and low-friction interface between the sleeve and the rope. The outside is coated with a plastic/rubber coating of some kind (I think some kind of PVC, but not sure). The coating is aesthetic and also may let it slide around the branch union and protect the tree a bit better than it would if it weren't there.
The part that is important is the steel core -- I've tried some all plastic alternatives and they just don't hold up well or provide a smooth enough surface for the rope to pass through. You can more easily find non-coated conduit -- just the naked steel core. Maybe you can coat that with plasti-dip to protect it and provide the same kind of properties that the rubber coating provides on the stuff we buy.
Also, there are a few different varieties of liquid-tight conduit. As you can see on my
flickr site, I've tried another one and it works okay -- not as well as the black ultra-flex, but it worked. I'm not sure at all what you mean by shower hose, but the ultra-flex that I use is very flexible and does not "remember" or hold it's shape once deformed. The Allflex (the alternative conduit I've tried) does hold it's arc shape, which actually seemed to make it more difficult to set and keep in the branch union.
Do let us know if you try something else with success (or failure, for that matter). Sorry to be pedantic, but remember to go low and slow when you're testing new stuff -- even though this doesn't seem life-support critical, you never know how things might backfire on you . . .