Do you mean the letter name (A,B,C) or the letter sound (how it is used within words) or the word a (as in 'a' cat)?
In British English the letter sound a is pronounced in different ways depending on the word.
The word a is pronounced in two ways. Either a as you mention above in ace or major, or an 'uh' sound (not as in are or father, more of the schwa sound).
The name of the letter A is pronounced as in ace or major.