Nutcracker?
Dark chocolate brown, boldly speckled with white flecks; blackish wings and tail; -conspicuous- white under tail-coverts and white corners to the tail tip. Long, heavy, blackish bill
Occasional/very rare in UK, Spain, France, Neths, Italy (but my bird guide is an old (60s) print.
However, I suspect you meant "brown, mottled with another brown"
Gull is quite a distictive body shape and you've eliminated the thrush shape. I've offered you a corvid-shaped suggestion: are there any other types you can think of which might be a better fit?