Hi Jayandjay,
For just birth certificates you are better off using
https://www.gro.gov.uk/gro/content/certificates/login.aspYou can get English and Welsh birth certs from 1837 - 100 years ago, and death certs 1837 - 1954 digitally, at a cost of £3 each, delivered immediately to your computer. Irish ones are free at
https://www.irishgenealogy.ie, and Scottish ones are on Scotlands People with a subscription.
Ancestry.co.uk can give you UK Censuses 1841 - 1921, the (redacted) 1939 Register, parish registers, gaol records, emigration records, poor law records, loads of stuff. Just not the actual birth, death or marriage certs. What you get on the Australian site, i dont know. or whether you would be better off with a World sub. Depends on what you are researching.