I really don't like that every sandwich or salad is absolutely slathered in mayo in Britain. I absolutely hate mayo more than anything, and I can never find a packaged sandwich without it. England has great mustard, but it seems impossible to get a sandwich with only mustard here. It's either just mayo, or mayo-mustard! It drives me crazy.
Where do you shop? That was one of the things that wierded me out the most when I first moved here--the sandwiches all seemed so prosaic. I bought a "chicken sandwich" at a supermarket on my first jet-lagged day without looking closely, expecting that it would have some kind of mayo or something, perhaps a bit of lettuce and tomato. Nope. Just chicken and bread. Same thing with cheese sandwiches (plain bread stuffed with shredded cheese). I find that a lot of the offerings at Tesco are like that.
But I don't think I have ever even seen a sandwich with mustard on it, let alone mustard by itself. Maybe in a slightly more upscale chain, like O'Brian's.
On the original post, I think that the sausage, bacon, mushroom, egg baguette with mayo seems perfectly normal. I'm sure I have had a similar combo in the States before I moved here, although maybe not on a baguette. In fact, just reading this makes me want one right now! I've grown accustomed to tuna salad with sweetcorn, but my Mom always made tuna salad with sweet pickle relish in it, so the sweet corn kinda takes the place of impossible-to-reliably-locate pickle relish. I don't like it on potatoes, though.
Years ago, when my Dad was recovering from heart surgery, the doctors put him on this low-fat diet and recommended that he make his own pizzas with low fat toppings and no cheese (using those ready-made pizza bases that were all the rage at the time). His favorite combo, which made the rest of us retch, was tuna and corn. The first time I visited Scotland, every pizza shop I went to (admittedly a small sample of places in Drumnadrochit, Inverness, and Kirkwall) featured this combo. Dad felt vindicated at last!
But at least in Scotland they put cheese on their pizzas...