they said as far as they were aware, given i was activating with AT&T in accordance with Apple's T&C's there was no issue on warranty.
AT&T has nothing to say about warranty issues, and AT&T reps are in general woefully informed about such matters. Apple has said that they do not support any attempts to unlock an iPhone, and if such an attempt renders the iPhone unfunctional, as has happened to some people, Apple will consider that to be an unauthorized modification which voids the warranty.
Furthermore, Turkey has authorised Apple outlets who claim to validly support my Apple extended plan warranty on my Mac.
Irrelevant. Mac sales are not restricted by country whereas the iPhone is. Further, it doesn't require any unauthorized modifications to use a Mac outside the country in which it is sold, whereas use of an iPhone with any cell network other than the one to which it's locked (or in France, a French network) does require unauthorized modifications.
Bottom line: an iPhone will not work as sold by Apple in any cell network but those for which the iPhone is sold (see above for a list). Anyone who attempts to modify an iPhone to work with another cell network will get not support from Apple and risks voiding the iPhones warranty.