Breakfast is the first meal of the day, one that often occurs before the household would have had the fire up and going long enough to cook a proper meal (dinner was around 2-3). Very often it would have been just cold left-overs from the previous day or might incorporate smoked/brined/etc meats that at most need to be heated up quickly on a skillet. Pork and sausage are perfect examples of smoked or salted meats. Kippers are fish found in English breakfasts today and are preserved. Fresh fish left over from yesterday's meal aren't exactly appetizing. Here's a list of 18th century cookbooks you can peruse to get an idea of where many traditional dishes developed from.