I just now encountered this strange expression in Graham Oppy's review of Owen Anderson's, The Clarity of God's Existence: The Ethics of Belief after the Enlightenment. The phrase occurs in this passage:
On the one hand, given that Anderson insists that he cannot be satisfied with ‘a sound proof that is extremely difficult to understand and that is knowable by only a few’ (123), it seems clear that his ‘program’ is bound to fail: for surely it is London to a brick that, if his ‘program’ could be successfully carried out, it would yield a proof that is ‘extremely difficult to understand and knowable only by a few’.
The phrase, apparently not in use in the U. K., is Australian and New Zealand slang for 'it is certain.' Explanation here.