Learn English – How to pronounce “new” in British English

The difference is that with bidet, as also with words like ballet, beret, buffet, café, cliché, and debris, in North America the stress falls on the second syllable, while Britain favors the first syllable in each of those words.

Wikipedia calls this “French stress”, and has quite a long list of these. A few of those are wrong, though, like for example négligée, which is stressed on the first syllable in both but which receives secondary stress at the end in North America.

One of the earliest instances of the verb ask occurs in Old English as acsian. As the etymological note in the Oxford English Dictionary explains, acsian and axian survived down to nearly 1600 as the regular literary form, but was supplanted in Standard English by ask, originally the northern form.

As you have observed, a(h)ks is still heard in regional dialects. There is no doubt, however, that the standard form is ask, both in writing and in speech, and it is that form that foreign learners of English should use.