There needn't be a 'British' rule as such; the 'britishness' of any entry is simply a factor for consideration by the judges.
When I was designing and building the original Clelands, I always looked at British products first, then European, then worldwide; quite a disappointing exercise back then.
No 5 looks wonderful to me; I wouldn't have specified a black stem because the only one long enough was made by Atom, supplied by the 'nice' Mr Kitching. No black anodised stems available, although they soon would be.
For this 'all black' build I had to dismantle assemblies and get each component powder-coated, and I dearly wanted to have the stem painted as well. I did consider having it done part-way down, but the problem was: how far down.
So, in short my dear Copperfield, the black stem is closer to the original concept than the original specification; I like it.