Gotta be careful here with my choice of words, or the "transatlantic language barrier" will kick in!
Dave, I'd agree with you up to a point. The biggest problems with valve seat recession over here have been with the "A-series" car engines as fitted to the Morris Minor, MGB Midget and the like. Standard procedure is, as you mention, to fit hardened (stellite?) valve seats.
There is also a theory that if an engine has covered, say, 40,000 miles on leaded fuel, it should do about half as much again on unleaded before any problems occur; I believe it's called "residual lead" or some such like. Goes without saying that on many older British engines, this will co-incide with a rebuild anyway. I understand that American car engines, apart from being more advanced than ours regarding lead-free technology (since you had it long before us) tend to be built to run higher mileages before overhaul is necessary. I put this down partly to the way things are Stateside, with your tendency perhaps to do longer runs at a steady speed than we have cause, or even space, to.
High-performance British bikes like the BSA Gold Star tended to be run on what we called 5-star fuel, above 100 octane. It was never available everywhere, and IIRC went out of use in the early 1970's. Its demise was due to the gradually-reducing amounts of tetra-ethyl-lead, which as you probably know is the halide added to gas to reduce detonation. Back in the day, any vehicle which had been run on that stuff, but only around town, would show a tell-tale white discharge around the exhaust pipe. The standard "Back street garage" cure for this was a fast thrash along the freeway!
The Morgan Owners Club produced some quite learned and interesting articles from one of their members on the subject of leaded fuel, and acceptable substitutes. I'll see if I can find it.