If you 'not sure' or only think the gull you are looking at might be Great Black-backed Gull, then it's probably a Lesser? They are certainly uncommon in Shropshire but every now and then one drops in at a roost or elsewhere...
The first clue of GBBG jizz is - it's a brute and smaller gulls tend give them a wide berth.
If you are still unsure and there's a few Lessers nearby, the comparison in size is a no brainer decision! The mantle is always greyish black and clearly darker than andy of the Lesser races. The presence of black on the bill may indicate it's a fourth winter rather than full adult winter.
In flight it is more ponderous with head and neck well protruding...
The barrel shaped body clearly seen here.
Off and away into the light...