Is "po-TAH-to" an acceptable pronunciation for "potato"?

I have lived in the UK for four years, yet I have never heard anyone say pot-ah-to, only pot-ay-to.

Normally, the word "potato" is exclusively pronounced with a long A (/ej/), in both American and British usage. Various dictionaries list only this pronunciation.

The Gershwin song intentionally uses a non-standard pronunciation as an exaggeration of the sociolectal differences between the two characters. According to Wikipedia:

The differences in pronunciation are not simply regional, however, and serve more specifically to identify class differences. At the time, typical American pronunciations were considered less "refined" by the upper-class, and there was a specific emphasis on the "broader" a sound. This class distinction with respect to pronunciation has been retained in caricatures, especially in the theater where the longer a pronunciation is most strongly associated with the word "darling."

Note that the rest of the song contains other clearly exaggerated pronunciations, including various regionalisms that would not likely be seen in the same speaker.