There are not many pronunciation differences between UK and US English, and there are some differences in the UK too, as has been pointed out.
The main sound for a 'closed' a, i. e. when it is followed by just one of more consonants, is as in 'cat, mat, hand, sand, panda' (in 466 of the 7,000 most used words which i examined).
An 'open' a, i.e. when followed by a single consonant and another vowel, has mainly the /ai/ay/, or long 'a' sound, as in 'male, mate, bacon, nation' (in 338 words of the 7,000).
But in southern England in 55 words a has the 'ah' sound of father even when 'closed', e.g. ask, blast, fast, last ...
In a few words a has other sounds as well: any, many, was, want, all, tall, and in quite a few words a is short before a single consonant and vowel (family, animal, salad...).
To begin with, it's obviously best to practise with words which have just the main sound short sound for a. But English phonics is not simple, especially not for vowels. That's why many children take quite a long time to become fluent readers.