I have seen the following species in the UK that were not in a 'bird zoo' (clipped and there just for general public amusement...) I call these my NOL (Non official list)
1.
SACRED IBIS (2012 Stanwick Lakes, Northants)
2.
BLACK SWAN ( First encounter at Dinton many years ago)
3.
HELMETED GUINEAFOWL (Various)
4.
MONK PARAKEET (Borehamwood with LGRE in 2015)
5.
HOODED MERGANSER (Radipole, 2015,2017)
6.
BUFFLEHEAD (Rodley NR)
It's of note for my own record, as a bird spotter in the UK that I have seen these unusual species in a wild environment.