Ginslinger's 'Aussie' question is a difficult one, because as Canadiankid says, there are lot of people in the 'old Commonwealth' countries, ie. Canada, SA, Oz and NZ, who are children of British Citizens and therefore can get full citizenship automatically. Think of them as Brits with funny accents
. These arrangements are reciprocated by many countries.
However Britain has a peculiar category known as an 'Ancestry Visa'. This is basically a no-questions-asked work permit followed by a fast track to British Citizenship for people who had at least one British grandparent. A large proportion of people in the 'old Commonwealth' fall into this category. This is usually not reciprocated by other countries (Australia does not do this for one).
The main reason why we get so many Aussies etc. flying over here, not to mention in many other professions, is because just so many of them fall into these two categories.