I quite like mayo but not half an inch of it !!! The only veggie sandwiches I can get at Tesco's are:-
egg mayo on oatmeal bread - seriously soggy bread too :(
cheese and tomato on white pap with said half inch thick of mayo
just cheese on wholemeal - which is ok ....
Asda do a nice egg salad sarnie but they put cream in it !!!! (now, I cannot understand cream being even wanted in a sandwich) or egg & cress with loads of mayo on oatmeal bread (once again, a soggy mess).
Greggs do an egg mayo on either white or wholemeal bread but since I usually find a bit of eggshell in there too, it's a bit offputting.... The only cheese sarnie they do comes smothered in branston which to me is worse than the mayo, doh !
In answer to your question Andy, I don't know why these sarnie makers insist on putting in huge quantities of mayo, I can't see that it's cheaper than a little marg, so I can only assume that they think the customer likes it that way !
Our local sarnie shops tend to put half an inch of marg on their butties, so I end up making my own .....