Constantly opening and closing beak.

There are numerous reasons. Heat so panting., something causing an obstruction like a fish hook, illness or building up to eject a pellet. Too many variables really. Did the bird look unwell? Did it go on to exhibit normal gull behavior ?

Could be a way for it to keep cool?

Hi Cin. The bird looked a bit unwell. It didn't move when a dog went near it. I just watched it for five minutes and then went on with my walk. I'll see if it's still there tomorrow morning. Maybe bird flu?

That doesn’t sound good at all,

This is the current Defra advice
‘You should call the Defra helpline (03459 33 55 77) if you find:

one or more dead bird of prey or owl
3 or more dead gulls or wild waterfowl (swans, geese and ducks)
5 or more dead birds of any species
We then collect some of these birds and test them to help us understand how the disease is distributed geographically and in different types of bird’

It’s also recommended that dogs are kept on lead to make sure they don’t pick up dead or dying birds