In my garden this afternoon.  At first I thought female Golden Pheasant but colours seem wrong.  Maybe it's just an aberrant Pheasant.

I think this is a form of leucism, as distinct from albinism, where specific pigments have not been produced creating this overall light colouration.

You get what might be regarded as the opposite effect of melanism where an excess of the melanin pigment has been produced leading to very dark, almost black birds.

Both of these are hereditary traits reinforced by the in-breeding that occurs in most game birds.