Since you're already booked to visit GR@RHR, the simple answer is to go for the Prestige menu at ninety pounds. As others have said, the dishes are never less complex in any restaurant, they are simply more numerous and smaller in quantity. And it's more of an experience to be served more dishes, you get the feeling of being spoilt, and why not go the whole hog the first time, then go ALC or set lunch subsequently. You will find that a fair proportion of punters at GR@RHR have the Prestige menu even at lunch-time. And these tasting/gourmand/degustation/prestige/exceptionel menus are never set in stone, the piper payer can ask for alternatives for a dish, we were served a lovely risotto with girolles and cep instead of the standard fois gras.