Many phoneme combinations do not work with AT&T UK English while they work with AT&T US English: For example, the following regex/phoneme works with AT&T US English but not AT&T UK English:
m aa r - t iy 1
The problem can be seen by testing the phoneme before usage.
However, while not correct or appropriate, if the phoneme is changed to something like m aa r - th iy 1 or m aa r - b iy 1, AT&T UK English will pronounce the phoneme problem free. This is just one example, but I have noticed many phoneme problems like this with AT&T UK English.