Ridgway's Cackling Goose Branta hutchinsii minima
These two individuals were in a private wildfowl collection and their keeper informed me of their identity. This form can often be much darker-looking than these examples, but it seems that there is quite a bit of variation among wild populations.
captive Ridgway's Cackling Geese, private collection in Wereham (Norfolk, UK), 5th May 2017