Just remember we don't all live in London, a common mistake:) And Cockney and Australian are not the same at all:) Also remember to study what area of Britain the character comes from. There are hundreds of different dialects here. I, for instance, speak with a completely different dialect from people who live in a town just 8 miles down the road. A few more miles and I have trouble understanding them at all:) Another thing to remember, to most Englishmen all Scots, Irish and Welsh sound the same, but they don't to each other:) Also It depends on how old the character is in some places. I am from Yorkshire and there are places where even generations sound differently, in some places there are old men who are completely incomprehensible, using very broad accents and dated vocabulary, whereas the younger generations tend to have a more understandable accent.