I haven't had or seen Australian toaster biscuits in a LONG time. However, my recollection is that they were softer than a Thomas' English muffin, and they were almost impossible to get the crunchy surface, which I like, but which Tim said he didn't care for. However, the ATBs were rather similar to most other store bought English muffins.
I just read more than I care to about the differences (including crumpets). Wikipedia tells us (not so helpfully) that "English muffins are sometimes referred to simply as muffins in the United Kingdom" (kinda like the joke that, in China, Chinese food is simply referred to as "food"), while "[t]he U.S.-style muffins (a sweet quickbread) are sometimes referred to as American muffins. . . ."