The phrase all Lombard Street to a China orange and its variants denote a near certainty.
—Synonym: London to a brick.

The term China orange denotes the sweet orange, the fruit of Citrus sinensis, originally brought from China. In Food: An Authoritative and Visual History and Dictionary of the Foods of the World (1996 edition), the American journalist and author Waverley Root (1903-82) wrote the following about oranges:

They were rare in early Tudor England but a trifle more familiar by Elizabeth’s time. […] By the time of Charles II the fruit was so common that “orange girls” sold it in theaters, and, at a slightly higher price, themselves.

Therefore, China orange came to be frequently used figuratively to mean anything of minimal value; for instance, in Newmarket: or, An essay on the turf. Containing, amongst other grave and weighty matters, a parallel (though not after the manner of Plutarch and Mr. Spence) between Newmarket races, and the Olympic games; very proper to be had in all pockets at the next Newmarket meeting (London, 1771), the English Church of England clergyman and schoolmaster Philip Parsons (1729-1812) described “the betting-post”:

This is a spot, where the company assemble upon the important business of forming their betts [sic], clustering together like the swarming bees […].
[…]
What a busy and active spirit reigns among that thronging croud [sic]! What a confused hum of mixed and earnest voices! And yet, though I call it confused, it is what Addison, by a bold and just figure, would call a regular confusion; for mark the ready pocket-books and pencils—“Squib against Janus, ten guineas to eight.”—“Done, Sir, done.”—“Camillus against the field, for a hundred guineas.”—“I am your man, Sir— agreed.”—“Five hundred to four, Gimcrack against Ranger;— did you take me up, Sir?”—“I did, my Lord;”—“very well.” “A hundred pounds to a China orange upon Eclipse;”—and thus, tho’ with the greatest confusion, yet with the punctual regularity of a merchant in his counting-house, is every bett carefully set down, and registered.

Lombard Street in London—so called because originally occupied by Lombard bankers—still contains many of the principal London banks. (Rue des Lombards in Paris has the same origin.)

The phrase therefore expresses a fanciful bet wagering the wealth that is available in the street’s banks against something of trifling value. In its earliest known occurrence, from The Citizen (London, 1763), a farce by the Irish playwright and actor Arthur Murphy (1727-1805), the comparison is between all Lombard Street and an eggshell; George Philpot is a reckless young man, Buttons and Gallows are the horses:

– Corinna. But can’t you let your coachman drive?—
– George Philpot. No, no—See me mount the box, handle the reins, my wrist turned down, square my elbows, stamp with my foot—Gee up!—Off we go—Button, do you want to have us over?—Do your work, do—Awhi! awhi!—There we bowl away; see how sharp they are—Gallows!—Softly, up hill [whistles.] there’s a public house—Give ’em a mouthful of water, do—And fetch me a dram—Drink it off—Gee up! Awhi! Awhi!—There we go scrambling altogether—Reach Epsom in an hour, and forty-three minutes, all Lombard street to an egg-shell, we do—There’s your work my girl!—

The Irish poet Thomas Moore (1779-1852) used another variant in Tom Crib’s Memorial to Congress (London, 1819), but noted that the more usual form was Lombard Street to a China orange:

                                        Cries of “Gregson”
Brought Bob, the Poet, on his legs soon—
(My eyes, how prettily Bob writes!
Talk of your Camels, Hogs, and Crabs,
And twenty more such Pidcock frights—
Bob’s worth a hundred of these dabs:
For a short turn up at a sonnet,
A round of odes, or Pastoral bout,
All Lombard-street to nine-pence on it,*
Bobby’s the boy would clean them out!)
* More usually “Lombard-street to a China orange.” There are several of these fanciful forms of betting—“Chelsea College to a sentry-box,” “Pompey’s Pillar to a stick of sealing-wax,” &c. &c.

Thomas Moore’s book refers to the English professional boxer Tom Crib (1781-1848), as does Pancratia, or, A history of pugilism. Containing a full account of every battle of note from the time of Broughton and Slack, down to the present day. Interspersed with anecdotes of all the celebrated pugilists of this country; with an argumentative proof, that pugilism, considered as a gymnic exercise, demands the admiration, and patronage of every free state, being calculated to inspire manly courage, and a spirit of independence—enabling us to resist slavery at home and enemies from abroad. Embellished with a correct and elegant engraved portrait of the champion, Crib (London, 1812); Tom Crib became British champion in 1810 and World champion the same year after defeating the American boxer Tom Molineux (1784-1818); the following passage from Pancratia is about the return fight between Crib and Molineux, which the former won in the eleventh round:

Saturday, September 28, 1811, the second battle between our renowned Champion, Thomas Crib, and the athletic black, Molineux, took place at Thistleton-gap, in the parish of Wymondham, in the county of Leicester […].
                                                                                            THE BATTLE.
[…]
9th [round]—Lombard-street to a China orange; Molineux was dead beat, and only stood up to encounter Crib’s death-like blows; he run in, Crib met him with his left hand, and broke his jaw, when he fell like a log; he did not come to time within half a minute, but Crib’s appetite was not to be satiated; he gave away this chance, dancing a hornpipe about the stage, and after hitting his opponent half down, and up again, floored him.