What is the typical composition of English Breakfast tea?
Well, blending is not easy and to do it in order to have a consistent product it requires quite the accurate nose. There is a reason why blender (for whatever product) is a very specialized job requiring natural disposition and years of training. 

And of course, the final product composition from both big and small companies is a very well guarded secret.

Here are some suggestions, you should try it out and see what you like. 

Assamica's Indian version of English Breakfast

Steepster forum (Tea lovers) discussing their own blends