"Question about Aussies - Do you use metric and Imperial (or US) units interchangably? (Oops I just betrayed my national origin. )"
Mentioning Imperial units instead of English units pretty much gives away an origin in a British Commonwealth country. I am Canadian but I am currently living and working in the USA. Americans use English units and the big distinction is in the US Gallon.
I use both systems, almost interchangably. My education was in [SI] Metric units, and my current workplace uses [US] English units. I have no idea how many times I have used numbers like 0.0254 or 0.225 in my calculations.
ADDIT: When I think about it more, I was expressing weights and body mass in kilograms and pounds, but people from the UK use stones, right?