(1) You cannot shoot anything unless you have permission to do so on that piece of land. This isn't automatic - the permission defaults to 'NO' so you have to actually have a (preferably written) 'YES'
(2) You will have to demonstrate that these gulls are a nuisance or a health hazard in line with the DEFRA licence's terms and conditions. Your say-so isn't enough. You can't simply go out and use a whole species for target practice, inviting though they might be.
To illustrate this, I was asked to clear the roof of a large factory in Westhill a couple of years ago. The local council refused me permission because I had not demonstrated a clear need.
Sorry.