Now for a number of reasons, part sentiment, I salt my spoken language with Australianisms. Well, it's London to a brick that I will be pulled up from time to time both younger and recent Australians. What do you mean, they say in puzzled terms?
London to a brick, you say? It just means to bet on an absolute certainty. I don't know when it first came in, although my memory was that it came from a race caller. How very Australian! I bit like betting on two blowies crawling up a wall!
That would fit with this account, although it appears from Bob's advice that the phrase changed a little. I have never heard it with the word "on" added. Well, London to a brick, I'm probably wrong!
For the edification and perhaps confusion of my international visitors, kvd kindly referred me to this dictionary of Australian slang.