Studying /ˈstʌdiɪŋ/, spelled s-t-u-d-y-i-n-g. It is study plus -ing, added directly with no letter change. The y stays; the y-to-i switch applies before endings like -ed, never before -ing.
I am studying English tonight. Say the ending as /iɪŋ/, a small glide before /ŋ/. Studing with one d is not a word.
She is studying for her exam. A natural use in the present continuous, with the stress on the first syllable, /ˈstʌdiɪŋ/.
We were studying together last night. Say the ending as /iɪŋ/, a small glide from /i/ into /ɪ/ before /ŋ/. Keep the two vowel sounds distinct so it does not flatten to /iŋ/.
There is no British versus American difference here. The spelling s-t-u-d-y-i-n-g and the pronunciation /ˈstʌdiɪŋ/ are correct in both varieties, so one form works everywhere.