I have no problems with historic sites meeting current health & safety regs. At least at Williamsburg the modern "improvements" are as out of site and unobstrusive as possible.
And I understand, too, why most recipes are adapted and modernized. They are, after all, in business, and have to meet patrons' taste requirements.
But when it comes to something like Welsh Rarebit, or other dishes that modern palettes would not find objectionable, there's no reason not to go with historic recipes (or, actually, reciepts, as they were then known).
Other places do that. For instance, the stewed tomatos dish served at Michie Tavern, near Montecello, is indistiguisable from the one served at raceday breakfasts in Kentucky as early as 1781. And, of course, venues such as City Tavern, in Philadelphia, serve only historically documentable dishes.
let alone the availability of modern ingredients at supermarkets versus what was purveyed in King George's time.
A common misconception. Due to the influence of Hollywood and popular "historic" novels, we tend to think that food in the 18th century was all plain and rustic, when, in fact, much of it---especially among the landed gentry---was very sophisticated. There weren't many foodstuffs available to us today that our forebears didn't have.
The big changes aren't so much with what's available, but in the convenience, forms, and time of year. For instance, looking again at Michie Tavern's stewed tomatoes, their recipe starts with canned tomatoes, whereas Bill Whitlock (well, his cooks, actually) would have had to first cook the tomatoes (more likely, cook them longer than the 15 minutes Michie specifies), then use them in the final recipe. And the dish would only have been made in the summer.
Kind of reminds me of the perennial argument of "fresh from scratch" vs. "convenience."