• Crumpets are always made with milk, but English muffins never do. (Though there are a few recipes that beg to differ.)
• Crumpet batter is just that: a loose batter. English muffins are made from a more firm dough. (Sometimes.)
• Crumpets are made only using baking soda, where as English muffins are made with yeast or sourdough (and occasionally a little baking soda to help things along).
• Crumpets are cooked only on one side, so the bottom is flat and toasted while the top is speckled with holes (delicious, butter-absorbing holes...). English muffins are more bread-like and are toasted on both sides. (This one is actually a definite!)