You just brought back great memories of the crispy bacon sandwiches with mayo and lettuce I used to have as a child in NC.
Unfortunately, a bacon butty is a massive disappointment. They are made with back bacon, which is more like an over cooked piece of ham. Not crispy at all. No mayo, no lettuce, just Some kind of gross sauce that tastes like bitter BBQ sauce. You aren't missing anything.
Certainly give marmite a go, you might find a condiment for life. I think it started out as the yeasty remains at the bottom of the Guinness barrels.