Even when the Americans do pronounce ate short, they typically use a different monophthong than the British. The British typically use epsilon; the Americans e, in which the tongue is a bit closer to the hard palate and more tense, creating greater vocal tract constriction. As John Algeo states in British or American English? A Handbook of Word and Grammar Patterns:
eat/ate/eaten : The British preterit is typically /εt/, the American /et/. In American, /εt/ is nonstandard.