I use not bad as an American to describe food all the time. Though it is a more tempered compliment than the British use. Not bad for me, means kind of good or surprisingly tolerable. Not bad meaning very good feels like just as bad of an abuse of language as calling a very good food amazing. If I were to fetch you a glass of water and you said that it wasn't cold, the first instinct is that was room temperature not that I got hot water out of the kettle. In the same sense, not bad implies middling or mediocre in taste.
I feel like overall it represents a trend towards understatement and adversity to strong compliment giving among Brits, whereas Americans are quicker to reach for extreme language and are much more forthcoming with praise and criticism alike.
