I've seen some numbers and peoples reactions to those numbers, although not always explained correctly, so here is some info from DIMIA, for the year 2003, the latest I can find.

Actual figures for those British Migrants that do return to the UK.

A total of 12,508 UK Born Migrants arrived in Australia in 2003

The departures figures for ALL British migrants permanently leaving during that year were:
Those having stayed less than 2 years: 685 left the country, to return home (about 1 in 20 of the number arriving)
Those having stayed between 2 and 4 years: 688 left the country, to return home.

If those figures are average, then about 90% will stay for 5 or more years, and 75% will stay for life.

Overall, they reckon that about 25% will return to the UK at some stage, but many of that % will stay here for the majority of their lives.

Some of those that return, do in fact come back again.
I am in that category, as I returned to the UK, never to set foot in Australia again, but after 4 years back in England, I returned to Australia

I have been asked why the numbers sometimes seem higher than this.

My view is that the vast majority of migrants no longer use the forums once they are settled.

Therefore you will tend not to get the true average opinion of ALL migrants.
You will mainly only get opinions from those that are still not yet fully settled, and are still using the forums to keep in touch with other British people.
Obviously there are exceptions to this, as some of us are just addicted