You can practice with Americans and Australians too! There are no significant differences. We all speak English. We all understand each other. It's the same language. Try a Canadian, or New Zealander. What IS British English anyway? Are you thinking it's a particular accent? There are more differences between the various regions of England than there are between standard American English and British English.

I think it is a good idea to practice with someone in the UK if that is where you will be using the language. But as MoiraWendy says, there is a wide range accents within the UK and it would be surprsing to hear a foreigner to speak with a Scottish or Irish accent (unless you plan to spend a lot of time in those places.) The differences are similar to the differences between Castilian and Latin American Spanish, in terms of pronunciation and vocabulary. For you to speak with a Scottish accent in London would be like me speaking with a Rioplatense accent in Madrid -- it's understood but unexpected.