Bufflehead Bucephala albeola
An extremely rare vagrant to the UK from North America. Records are usually tainted with the possibility that they involve escaped birds rather than wild vagrants, and certainly some records have involved birds bearing an aviculturalist ring while others have been very tame suggesting a captive past. The only photos I'm able to offer you for now are these captive birds.
captive Buffleheads, Martin Mere (Lancashire, UK), 26th December 2014