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

There's no standard blend for English Breakfast tea, and different tea companies may have their own flavor profile they want to achieve. Generally, the goal is to create a more robust tea that goes well with milk.

Traditionally, teas from Assam, Ceylon and Kenya are used for the blend. However, today, teas from more locations are used. The benefit of blending teas from more regions is that the 'average' flavor will stay more consistent throughout the seasons and years. For the tea taster, it's therefore possible to make just small tweaks to maintain that same flavor and aroma profile.

In the tea community, you've people who like consistency and go for tea bags and blends, and you've people who like diversity and go for single-origin.