I am living in UK and if you are thinking of going to London to improve your English, forget about it. There is almost no English people living in London, so the last place I'd recommend you to go would be London. I've been living in London for almost 2 years, now, I am living in other place and the difference is huge! If you want my recommendation, go to any place in the South of England. Avoid any northern than Bristol-Oxford-Cambridge as they speak a not-easy-to-understand English
.
I am checking the pictures of NZ and it is really amazing.
Regarding Australia, it is not that bad
. I have Australian friends, and they say that the "dangerous" animals, insects, whatever, are not in the cities, and you have to go deep into the wild to find them. Of course there are a lot, but you won't be in danger in the city. For example, in the north coast of Australia is where most likely to find deathly jellyfish and white sharks, but it is very well known, so nobody goes there (except surfers
).