if you talk in Queen's (or Royal or whatever you call it) English
you call it RP - en.wikipedia.org/wiki/Received_Pronunciation ;))
as for the Polish ę and ą (but not only)
there is a notion in phonetics of Polish which is called "hypercorrection"
(although it is not entirely similar to the notion in other languages),
which states that using such sounds like ę, ą, voiced plosive + ł, and some other
is incorrect. some linguists even treat it as an error, while others argue that something
that has "correction" ("poprawność") in its name should not be treated as an error.
anyway, because of that, you can easily hear people saying "dzienkuje" instead of "dziękuję",
and generally speaking dropping final ę for e and middle ę for (usually) en or (seldom) em.
there is an important difference as "dzienkuje" (ending with -e) is 3rd sg. while -ę is 1st sg.
however, usually the context tells you which is which and eventually you can use both.
nonetheless, final -ę is the correct form.
it's, up to a point, similar to English 'dunno', 'gimme', 'lemme' etc.
a simplified version, used in spoken language but not entirely correct.
it does not vary in different regions as far as dialects are concerned.
it may vary in urban and rural regions though.
sry for eventual mistakes - I'm 3 sheets at the moment...