Here, have a cuppa
Let's say you're out to breakfast and you've made the decision to give up coffee, so you decide to order black tea for your caffeine fix instead. (Clever.) So you place your order and the server asks, "Well, what kind, exactly?" And that's when you notice that there's a full list just under the "Black Tea" portion of the menu. We've already talked about what makes Earl Grey tea different from other black teas, but it's time to address the difference between English, Irish, and Scottish breakfast tea. So next time, when that server is tapping her pencil on her notepad, waiting for you to answer, you can make an informed decision. Pinky out.
The first important thing to keep in mind is that all of these breakfast teas—English, Irish, Scottish, and Earl Grey—are black tea blends. Your basic black tea is typically made from the leaves of different varieties of a shrub called camellia sinensis, usually the Chinese variety plant and the Assamese plant. The exact blend depends entirely on the tea maker; there's never been a standard recipe put in place for what constitutes an English breakfast tea blend compared to an Irish breakfast tea blend. That said, there are some generally agreed upon attributes for each kind of tea, and the reasons behind that are steeped in history. (See what I did there? I'm sorry.)
English breakfast tea was originally a black tea from China, called congou. However, China imposed an embargo on congou during the Opium Wars. To deal, England's British East India Company began producing tea in Assam, India, which meant there was a point at which people began combining the congou and Assam teas. A few hundred years later, England began importing tea from Sri Lanka (Ceylon, at that time) and threw that in the mix. Nowadays, English breakfast is a blend of tea from Assam, Ceylon, and even Kenya. It's typically described as a rich, full-bodied tea, and it's meant to go well with milk and sugar. Of course, pairing well with a full English breakfast is of the utmost importance as well.
Irish breakfast tea is known for it's strong Assam component—likely due to the tea trade trends when tea became popular—which tends to add a malt-like flavor to tea. It's also a more robust tea than English breakfast. As someone explained in an online tea forum (yes, they exist): "I’ve always said English breakfast is a coax-you-awake; Irish breakfast is a kick-you-awake."
Still, Scottish breakfast tea is thought to be stronger, still. While the blend is similar to Irish and English breakfast teas, Scottish breakfast tea was likely blended specifically to overcome Scotland's soft water, which can sometimes taste a bit salty.
So, all of the U.K.'s breakfast blends basically consist of the same components, but trends in trade and water conditions can change those blends ever so slightly. That said, here's the most important thing to know: If you need the biggest caffeine jolt, go Scottish. They know what they're doing.