Albino Blackbird could well be the answer!
It looks pretty similar to this:
But almost completely white on the body and completely brown on the head.
To answer some of the Q’s from above:
Where was it seem? on the ground, in a tree for example..
I’ve seen it on the ground, in a tree and in flight.
Did it have a long or short tail?
Short – so not an albino Magpie (unless it had lost it’s tail feathers.
And did it make a call or any sort?
Nope.
It’s likely to have a leucistic condition so it’s normal colourations aren’t present.
Also did it appear to be alone or where there other birds present?
Mostly seen on its own (in flight and in a tree) but when I saw it on the ground today it was hanging about with a Rook.
No luck with the RSPB bird identifier but the Bird ID picture of the Black Headed Gull is pretty close as far as the colouring is concerned. However, I know what a gull looks like and this mystery bird is definitely not a gull.
Thanks for heads up about the Albino Blackbird SA.