I presume this is a fairly new model with the US wireless keyboard? I note from the pics on the Apple Store that thekey is marked with'delete' on that keyboard rather than the backspace arrow which appears on the British keyboard (and every other one I took the trouble to enlarge).
This question came up several months ago, but I don't know if the OP on that occasion ever got a fix to make it work as backspace and require the Fn key for forward delete.
You might try going to System Preferences > Language and Text > Input sources and change the selection to British. See if that changes it. That's not a long term fix, because other functions will be different, too.