I've seen all of these bikes in the flesh, though I think I've only actually ridden on a Rupp and a Bultaco, briefly.
I gotta say, though Mr Britten had some good ideas, I cannot consider his brand in the same category as these others, who manufactured many thousands of bikes over much longer period - I consider him more of a "boutique bike designer", albeit a brilliant one.
I recall being a teenager in the 70s and lusting after a Laverda Jota, though. Good thing I didn't have the ability to get one then,as I'd likely have killed myself on such a machine