What's this white pigeon-sized bird, with dark grey spots ?

.
The text description of the bird ("X") is from about 3 weeks ago.

3 weeks ago, X was 90% white with dark grey spots.

Each spot was the size a 5p coin. Each spot was about 2 inches from any neighbouring spot. So the bird was about 90% white.

When the wings were folded, the trailing edge of each wing was the same dark grey as the spots.

Today:  The spots are disappearing, so I'm assuming that X will eventually be all-white.

The length and height of X has always been similar to its friend, which is a normal UK grey town pigeon. But the pigeon is more sleek.

By "sleek" I mean that that the largest measurement round the body is smaller for the pigeon than for X.

When both birds are together, and the pigeon is startled, both birds fly off in unison.

But if X is alone, I can gradually edge to within half of the distance that would startle the pigeon. I keep my eyes averted, and I walk only sideways or backwards, at about 2 inches per second.

I would be grateful if someone could identify this bird.

Thanks

Footnote:  The spell-checker is based on US spelling. So "neighbouring" was marked as incorrect. Removing the "u" removed the red underline.
.