This is an interesting subject.
I've heard that English single triggers are lousy, from more than one source.
Note that production guns from around the world seem to have reliable single triggers. The technology is fully developed for PRODUCTION guns, where a standard design made from interchangeable parts is accepted and works well. Hell, I have a lowly SKB/Ithaca with a Jap copy single trigger that's 100% reliable. It certainly can be done...
Perhaps, as a guess, maybe single triggers aren't so well adapted to one-off style gun making? You can't expect screws (excuse me, pins) to interchange on a Brit best, perhaps it's difficult to make the trigger close enough to design specs for good function with traditional techniques.
This may be one case where machine made precision is required for function?
