Several species of gull (or "seagull") occur in Britain. Those most often seen in coastal towns are Herring Gull, Lesser Black-Backed Gull and Black-headed Gull; they are a quintessential part of the British seaside. As with many seabirds, all these species are in fact declining in the UK, for example Herring Gull numbers have declined by more than 40% since 1970.
Some of these species are very adaptable in their diet and this brings them into conflict with humans as they are well-placed to take advantage of people's messy habits by feeding on food waste and scavenging. To deter gulls from nesting you should not feed them, and make efforts not to leave waste in accessible areas. Several companies sell devices to deter gulls from nesting on roofs; these should be erected safely well in advance of the nesting season.
As with all birds, the law protects gulls and their nests. However there are special provisions to allow action to prevent spread of disease and ensure public health and safety. Defra (the government Department for the Environment, Food and Rural Affairs) can provide more advice on this.