Well,I owned a 1967 BSA Shooting Star,441 cc single,in 1968 to1970.NO leaks.No issues until 2nd gear went away from power shifting to get the big wheelies going.
I owned a 1974 Triumph Trident from 1978 to 1998.NO leaks.Head gasket failure due to semi-clogged carbureters causing an overheating episode.Bad gas.
All of my friends rode British bikes at one time or another,some,but not all, leaked.Some of the guys had "trued" the case joints,I admit it.
In my personal experience,English bikes were not chronic oil users.A lot of them had chain lubrication using engine oil,these could over do it and be messy,I suppose.My Enfield is leaky in a seeping sort of way,never drips unless a new leak comes along.
I would say that what we have is somewhat less than what we would have obtained from England.I believe this is obvious.They only resemble the old English bikes.
I find no little irony in noting that the worst bike I have ever owned has a British heritage but was not made there,and that the faults of it tend to be attributed to British bike history when the present firm have cut every corner possible,until recently,resulting in a product the Brits would have shunned.
Still,I have an attatchment to Jolly.I really just want to make it better and keep it.