Is there a big difference between english spoken in Britain and in Australia?
Not really. Standard British English and standard Australian English are pretty similar, give or take a few minor slang expressions.
As Coligno has pointed out, one of the biggest differences between varieties of spoken English exists <em>within</em> the British Isles. In fact, the differences are so great that if you heard four informal conversations in pubs in Liverpool, Newcastle, Glasgow and Belfast, you would be forgiven for thinking that these four set of people were actually speaking totally different languages from each other.
By contrast, the more 'neutral' accent of SE England and that of most Australians are in fact fairly alike, so much so that many people from other countries - English-speaking countries included - might easily confuse a neutral accent from Sydney with one from London.
'Neutral' British English, on the other hand,
While there are some minor differences, British and Australian people have no difficulty understanding each other and holding full conversations on any topic.
Thank you all. That's exactly what I wanted to know befory my way to Australia. :)
- and please ignore that last bit. That sentence isn't going anywhere...