Yep, Kafue Flats Lechwe - and obviously the UK zoo line is still throwing up the occasional white individual!
Deceptively complicated question! Traditionally it is considered a subspecies of Southern Lechwe (Kobus leche) under the name Kobus leche kafuensis - all lechwe are in this species except Nile (K. megaceros). Recently it has been suggested that the various 'K. leche' subspecies are distinct species, in which case it would take the name Kobus kafuensis (and I think the name 'Southern Lechwe' falls by the wayside, as all the subspecies have their own common names - Red, Black, Upemba, Kafue Flats and the probably-extinct Roberts'). Certainly there is evidence of good genetic separation between them, and a degree of physical difference as well - kafuensis has bigger horns on average, and much more black at the top of the legs than other 'red' lechwe.