I've had a few brit bikes from the 40's to the 70's and not had to relearn which hand does what... On older british bikes it was only the foot controls that are 'reversed' - gear shifter on the right, rear brake on the left. But in common with most bikes the handlebar controls are the same - front brake on the right with the throttle, clutch on the left. 'Modern' british bikes (new Triumph etc.) have completely standard control placement.
Some bikes (certain OSSA (Italian) engines for example) had a gear shift shaft that goes all way through the casings so you could choose which side the lever went on, but as standard they still had left clutch, right throttle/brake...
The only time I have ever seen a front brake lever on the left is on US spec bicycles (or on hipster bicycles because the owner thinks the US handing of brake levers 'looks cool'). And, the only bike I am aware of with a left hand throttle from the factory is an Indian ("America's first motorcycle") or as an option on very early Harleys. As far as I know, anything else with the throttle on the left is either an intentional modification (i.e. because of a disability etc.) or a restoration mistake.
Of course, with real veteran bikes (e.g. pre-1915), anything goes with regards to control placement, and there were a LOT of things to manually control back then.......