It seems there was one in Tufnell Park a couple of years ago, might have wandered over our way?
Probably leucistic rather than albino http://en.wikipedia.org/wiki/Leucistic
As far as I know leucism exists in all species, so it's perfectly possible. It's quite rare so good on her for spotting it.
It looks like you're new here. If you want to get involved, click one of these buttons!