Waitrose honours the price of the original item, if subbed. I think they honour offers in 3 for 2's etc too (but I always add picker notes to tell them what to do in those circumstances).
My local(ish) butcher that delivers just finds random stuff costing a bit more from the deli shelves (so I had some smoked garlic salt and some chutney in place of veggie burgers in the summer)!
My local Coop (non delivery), will often let us sub for their crazily cheap junk food offers (like pizza, beer and ice cream for a fiver).