Most mammals to a greater or lesser extent, have the occurence of albinism (if that's the right word). In wild populations these individuals are unlikely to survive beyond infancy due to being more vulnerable to predators or to being rejected. I'm not sure how common major variations in colouring are in the Mainland population of Greys, nor if there are pockets of populations of other varieties of squirrel. But albinos do survive to adulthood on occasion, so this may explain your white squirrel. Were you close enough to see if it had red eyes?
Yes, albinism was was my first thought - I didn't get a close enough look to tell properly - certainly it was fully grown.
It was a very bright white and, although I didn't see anything unusual about the eyes, pink would have been sufficiently contrastive not to have been noticeable at a distance. Other (remote) possibility (after googling) is leucism.
There are pictures showing the difference between the two here...
http://www.bbc.co.uk/news/uk-36919221 [ Rare albino squirrel pictured in Sussex garden ]
and
http://metro.co.uk/2015/10/26/photo...ne-of-four-white-squirrels-in-the-uk-5462498/ [Leucism]