There's no such thing as a British accent. Britain is made up of England and Wales, and Great Britain is England, Wales and Scotland. Also, when people say 'Britain' they mean 'the UK' which includes Northern Ireland. All four of these countries (England, Scotland, Wales and Ireland) have very different accents, so you'll have to be more specific...
As Taylor mentioned, there isn't one single "British" accent, although most of the world knows Britain for it's "Queen's English" (also known as received pronunciation).
There are varieties on the Australian accent too, especially considering the ethnic variety on the island.
However, general differences can be determined. This video is interesting and useful:
As for Iggy (I assume you're talking about Iggy Azaela), there seems to be a question mark over her accent.
I think it'd be fair to say that, due to her admiration and emersion in rap culture, she has developed a more East-coast American accent, especially in her music, although I doubt that's the way she talks in day-to-day conversations.
It might be perceived as "fake" for her to have adapted a whole new accent, but bear in mind that us humans are susceptible to picking up accents quite easily, it's a trait of our biological survival skill set, which helps us adapt to new environments by mimicking the behaviour and sound of those around us... Some people are more adapt to it than others though.
