I found their sizes are slightly bigger. I'm a UK12 in most boots and a US13 I a Nike runner.
My right size RMW Craftsman boot was an 11.
As you say, beat to check in store. I ended up wanting to change mine to a different sole. Although I got mine at Myer, their Collins St store was happy to swap. Their comment was "Doesn't matter where you bought them. They're our boots."
So long as it's obvious there were never worn.