The *descriptive* answer in British English is:
"learned" is used in phrases such as "a learned professor", in which case it is pronounced with two syllables.
Either "learnt" or "learned" are used interchangably in phrases like "I learnt a valuable lesson today".
The *descriptive* answer in American English is:
There is no such word as "learnt". Use "learned" always.
Whilst I agree with most of your post, I must take issue with the term "British English". In England, we do not speak British English, nor do we speak British Scottish or British welsh, we speak English, Scottish or Welsh, (other languages may also be spoken). I also understand that you may have been using the term to distinguish from American English, but this extra distinction in unnecessary. Please feel free to call the American version "American English" but refrain from using that awful and horrendously incorrect term, Thank you
