Yes, if a UK birder just says 'Scaup' it will almost always mean Greater. Lessers occur as rare vagrants, but even UK field guides habitually call the two 'Scaup' and 'Lesser Scaup'.
I, on the other hand, am a hateful little pedant and would put the full name every time if I intend one species in particular!
EDIT: Of course, I'd need to actually see (and identify) one of the little so-and-so's first...
I, on the other hand, am a hateful little pedant and would put the full name every time if I intend one species in particular!
EDIT: Of course, I'd need to actually see (and identify) one of the little so-and-so's first...