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England and NewZealand i duno

but i know many information for Australia
There are many different kind of english course, like 1 month, 3 month 6 month and 12 months!
The price is depends on the course length, if you are a Hong Kong resident, you dont need to have a saving in the bank.

You need to check the details the agents, like IDP
They can organise the school, the course, and homestay for you
But you are not able to work with that Visa... you need to get a working visa when you arrive

but if u want to stay in different state and different place...
i think working holiday visa is a better choice for you!
You dont need to study in any school but you are able to work and stay in Australia/ UK.

Working Hoilday Visa for Australia:
http://www.immi.gov.au/visitors/working-holiday/index.htm

Working Hoilday Visa for England:

www.ukvisas.gov.uk/servlet/Front?pagename=OpenMarket/Xcelerate/ShowPage&c=Page&cid=101872106