I don't think the total dish should be written in stone as appetites vary from day to day as do cultural differences across the nation.
However, there are one or two essentials without which, it is not a British breakfast. Bacon is the first and prime ingredient, it can be streaky, back,or any other part of a cured pig as long as it real .eat and not that reconstituted crap they serve up in left pondian establishments that has a closer relationship with chipboard than bacon, my preference is the huge rasher that you get in old fashion ed transport cafes, salty with acouple of little bones.
Fried eggs are a must, if you wan t scrambled by all means add a slice of toast and top it with scrambled eggs but they must have been cooked with butter not margerine. A couple of regionally appropriate bangers, beans, fried tom's, fried mushrooms, fried bread, hash browns seem to be common now but are not really British. Not last or least, black pudding is a must for me, I love the stuff.
If in Glasgow you can add anything dipped in batter that can fit in the deep fryer.
I can't continue...... must,,,,,,get,,,,, to,,,,,,, cafe,,,,now!
PS I almost forgot bubble and squeak a truly British breakfasty thing