Yes,I'd tend to agree with Ian,its more than likely some cage bird breeders cast off.See quite a few birds like that in S.W.Scotland where keepin cage birds(both legal and illegal)is still a popular hobby.At a feeding station near Castle Kennedy I've seen strange looking Chaffinchs,Greenfinchs,and Goldfinchs,all presumably breeders unwanted cast offs.
It looks to be possibly part leucistic but as Henry has said it could well be just a juvenile. Your photo is a little too distant to tell unfortunately.
If it has a plain brown head instead of the dartboard head of the adult and the rest of the bird looks the same, then it's a youngster. Quite a tricky time of year with all the juvenile plumages about! Thanks. Henry.