Often in to bet London to a brick, the Australian-English phrase London to a brick expresses the belief:
– (in horseracing contexts) that a bet is sure to pay off;
– (in extended use) that something is a very strong probability.
—Synonym: all Lombard Street to a China orange.
This phrase is more correctly London to a brick on, that is to say: the punter is so confident of winning the bet that he is prepared to put the whole city of London on a horse to win a brick, i.e., a ten-pound note.
BRICK: TEN-POUND NOTE
With reference to the colour of the Australian ten-pound note, which was a dull red, brick was colloquially used to denote a ten-pound note (when decimal currency was introduced in 1966, brick came to also denote a ten-dollar note).
These are early occurrences of brick used in the sense of a ten-pound note—with a pun on brick in the literal sense of a block of building material:
1-: From this advertisement for a bookmaker, published in The Standard (Port Melbourne, Victoria) of Saturday 12th June 1886—this advertisement referred to annual horse-racing events taking place in Melbourne, Victoria:
PUT DOWN A BRICK, AND PICK UP A HOUSE.
Double-CAULFIELD AND MELBOURNE CUPS—500 To 1.
V.R.C. DERBY AND CUP—200 To 1.
COMMISSIONS Executed on Principal Events.
J. J. BARTLETT, Jun,
Railway Club Hotel,
PORT MELBOURNE.
2-: From the Newcastle Morning Herald and Miners’ Advocate (Newcastle, New South Wales) of Thursday 23rd December 1886:
“MARTINI’S” CONSULTATION.
The drawing of “Martini’s” consultation on the Summer Cup will take place tomorrow afternoon. Those who wish to be present are invited to an outing by steamer, particulars of which will be given in tomorrow’s issue. We understand that the tickets for this consultation have gone off more rapidly than any of its predecessors, a fact alone which speaks volumes for the genuineness and fairness with which they are carried out. A few tickets still remain for disposal, and those who wish to “put down a brick and pick up a house” should not delay in sending in their applications.
3-: From The Referee (Sydney, New South Wales) of Thursday 5th May 1887—Wilcannia is a town in New South Wales:
The Wilcannia settling took place on Monday night. Nearly £700 was paid over, Mr. T. Byrne being the principal winner.
Don’t read “Barb’s” advertisement on page 4, nor put down a brick or you pick up a castle. Others that have done so complain—not much.
EARLIER BRITISH-ENGLISH PHRASES—AND CONJECTURES
I have found two British-English occurrences of phrases similar to the Australian-English London to a brick, which predate the earliest occurrence that I have found of the Australian-English phrase—these British-English occurrences are:
1-: From the account of a boxing match between Hall and Sampson, published in The Star (London, England) of Monday 5th August 1822:
Sampson received an echoing blow in the short ribs and wind, and went down wofully [sic] distressed—10 to 1. The poundage went round in vain, and a cockney called out “All London to a brick-bat!”
2-: From Shooting at Exmouth, published in The Western Times (Exeter, Devon, England) of Tuesday 29th May 1900:
On Saturday afternoon members of the Exmouth Rifle Company held their first shoot in connection with the monthly prizes and the Alston Cup […]. At the first range Sergt Beavis put on 20. On going back to 500 he placed another 23 to his credit, and it looked “all London to a brick” that he would come out an early first, but at 600 he dropped off, and Lnc-Sgt Carder, who had been shooting up to his usual reputation, succeeded in beating him by one point.
Therefore, perhaps (but these are only conjectures):
– originally, in British-English use, the image was of betting the whole city of London, considered as a mass of brick-buildings, against a single brick;
– when the phrase was introduced in Australia-English use, the original image became associated with the existing colloquial use of brick in the sense of a ten-pound note.
EARLY OCCURRENCES OF THE AUSTRALIAN-ENGLISH PHRASE
These are, in chronological order, the earliest occurrences of the Australian-English phrase London to a brick that I have found:
1-: From an advertisement for Hallam Cycle Company, published in The Daily Post (Hobart, Tasmania) of Friday 24th December 1909:
AT THIS FESTIVE SEASON OF THE YEAR ONE IS APT TO INDULGE IN FOND REMEMBRANCES,
AND WE THINK WE ARE SAFE IN BETTING
LONDON TO A BRICK
THAT ONE OF THE SWEETEST TIMES OF YOUR LIFE IS WHEN MOUNTED ON A
HALLAM CYCLE
2-: From an article about lawn tennis, published in The Sun (Sydney, New South Wales) of Monday 30th September 1912:
There are twelve singles in inter-State matches, and nine doubles, so it is imperative that players should be able to play singles. In Sydney singles are as rare as feathered frogs, and while this is so it is all London to a brick that Victoria will beat our best, and Queensland will continue to account for teams that are sent up under existing conditions.
3-: From the account of races that took place at the Hay Jockey Club, published in The Riverine Grazier (Hay, New South Wales) of Tuesday 16th October 1917:
The Shorts Handicap was responsible for Mr. J. T. Matthews’ Carbinier gelding Carcoo adding another victory to his credit from Veluvezoom. Had the latter got away on even terms with the winner it would have been London to a brick on the mare; as it was she lost many lengths, and was then only beaten by a short head.
4-: From the account of a match between the Imperials and the Rovers, published in The Riverine Grazier (Hay, New South Wales) of Friday 22nd May 1931:
The first round in the local football competition ended in the park oval on Wednesday, when the Imperials scored a runaway victory over the Rovers by 104 points to 15. […] As the players filed on to the field it was noticeable that the Imperials were very well represented, while their opponents were a weak and light combination, and it looked London to a brick on the Imperials. The game had not been long in progress when the superiority of the Imps was manifested, and they eventually ran out the easiest of winners.
KEN HOWARD
The phrase was popularised by the Australian racing commentator Ken Howard (1913-1976). For example, the following is from The Magic Eye of Ken Howard: Broadcaster Who Beats The Camera To The Finish, published in Smith’s Weekly (Sydney, New South Wales) of Saturday 19th April 1947:
Introduction of the magic eye camera to Sydney racecourses has made Ken Howard (2UE-ASB) the most talked-about broadcaster in the Commonwealth. In every instance, with one exception, where the judge has called for a photo, Howard has not only nominated the need for the camera picture, but has also given the correct decision the moment the horses have crossed the line.
[…]
In the La Perouse Handicap on Sydney Cup day, Bundagen and Arrowsmith went stride for stride in the final few yards. When a photo finish was semaphored, Howard commented: “They’ve called for a photo, but it’s London to a brick on Arrowsmith.” Bookmakers on the course laid 10-1 against Arrowsmith getting the verdict.
The following is from an interview of Ken Howard (who had just retired), published in The Sydney Morning Herald (Sydney, New South Wales) of Tuesday 1st January 1974:
A Howard trademark was his confident prediction of the winner in a tight finish—“It’s London to a brick on.”
[…]
Where did the expression “London to a brick on” come from?
“I don’t know, but I probably picked it up in the billiard halls when I was younger,” he said.
“I used to meet a lot of Damon Runyon characters *, listen to their talk and pick up some of their expressions.”
* The phrase Runyon character refers to the streetwise New Yorkers—bookies, gamblers, fight promoters, etc.—portrayed by the U.S. journalist and short-story writer Damon Runyon (1880-1946).
word histories 
The Australian ten pound note was indeed referred to as a “brick”, however when Australia introduced decimal currency ten shillings became a dollar and one pound became two dollars thus the ten pound note became a twenty dollar note. Its colour was red. The ten dollar note became blue corresponding to the blue colour of the five pound note.
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