Although the page is called My Ancestors it's not just for direct ancestors - you can enter any of your relatives. Indeed, if you were too look at my page you'd see that less than 5% of my entries are of direct ancestors (and if I were to delete them the change in my Match Potential would be barely noticeable).
So in one sense it doesn't make much difference when your direct ancestors migrated, because it's the relatives they left behind in Britain who are the links to your 'lost cousins' in Britain. Whilst the majority of your 'known cousins' may live in Australia, the vast majority of your 'lost cousins' will still be in Britain.
However it is true that the more recently your direct ancestors left Britain the easier it is to find those of your relatives who were recorded in 1841, 1881, and perhaps even 1911. If some or all of your direct ancestors migrated before 1841 then you might have to research a little harder, I'm afraid.
But difficult isn't the same as impossible!