Now I'm probably not a good example, and if you're sociable and friendly, you should be ok. But I did find it difficult, and it wasn't that easy just going up and talking to people in the bar. It is rather cliquey, to be honest.
I also have friends who adore Eastercon, and the other UK conventions, and get on like a house on fire. But IMO it's better to go with a friend or partner, or else with an intro.