See Also: Muffin History
The story is that an English baker, a certain Samuel B. Thomas, started making these flat chewy things in America over 100 years ago, from his mother's tea cake recipe. The English deny that they ever heard or saw anything like it until they were imported from America. Today you can find Thomas' English Muffins in most English supermarkets. Imported from America.
The curious thing is that 'muffins' in the U.S. are not anything like these so-called 'English Muffins'. (Maybe this was an inexperienced English immigrant baker's attempt to make crumpets* from a half remembered recipe of his mother's.) Muffins in America are 'quick breads' that is, made with no yeast, but leavened with egg and baking powder.
‘English Muffins’ are about 3 inches round and 1 inch high, yeast raised (basically a bread dough) and baked on a griddle. To get the proper texture when split in two they should not be cut with a knife, but should be split with a fork. The resulting rough texture gives them a certain crunchiness when toasted (and helps them hold gobs of butter and preserves).
They are an essential ingredient in Eggs Benedict .
* (What's a crumpet? That's another story.)