When we lived in Aberdeen, we quickly learned that eating fish and chips outdoors was dangerous. The seagulls would come to help themselves, and steal scraps. If you did not drop scraps, they would steal from you without asking. And the seagulls were Enormous. In Tobermory on the Isle of Mull, they wait around the excellent fish and chip van on the harbour, although they don't steal in the same way. Not yet, anyway.
Seagulls have been in the news over the summer for attacking pets and people, sometimes causing nasty injuries. I do remember news reports when we were in Aberdeen that if you had gulls nesting on your house, you needed to watch out for them attacking you if they had chicks.
The recent BBC programme Big Blue Live was very good, looking at life in the seas and on the coast around the UK and beyond. It had a feature about gulls, looking at one man who had taken steps to make them behave by waving sticks at them when they came from neighbouring buildings into his garden. This seemed to show them what was what, and their behaviour improved.
So, what information is out there about badly behaved gulls? What action can you legally take? What should you do if one tries to steal your chips? In addition to the sites below, I think it would be worth checking the website of your local authority - several appear in the first page of Google results.
RSPB on urban gulls
RSPB on gulls and the law
Interestingly, there is a lot of information here about deterring gulls, but rather less on what to do if you are attacked by one.
Skuas are known for doing the same - my parents tell stories from their visit to Shetland of getting too close to nesting sites of bonxies, as they are known there, and having to wave sticks in the air so the birds attack the stick and not their heads. The Skuas, by Robert Furness, has a chapter about skuas diving attacks on humans, but because I have only seen what is available in Google Books (not all of the book), I don't know if he has any advice on what to do if they do attack.
(I do know there is no such thing as a seagull, as there are many species of gull, but I am hopeful that you know what I mean if I use the word!).