The phrase to drive a coach and horses through something, and its variants, mean: to expose the flaws in something such as a law, a policy, an argument or a belief.

For example, the British Minister of State for Immigration, Robert Jenrick (born 1982), used the phrase in the House of Commons on Tuesday 11^th July 2023, during the consideration of the Lords amendments to the illegal migration bill—as reported by The Guardian (London and Manchester, England) of that day:

The immigration minister Robert Jenrick says in considering each of the Lords amendments MPs must ensure they don’t “drive a coach and horses through” what the bill is trying to achieve.

In the phrase to drive a coach and horses through something, the flaws in something such as a law, a policy, an argument or a belief are likened to holes large enough to drive a coach and horses through them.

In fact, the image of a hole large enough to drive a coach and six horses into it occurred in The Relapse; Or, Virtue in Danger ([London]: Printed for Samuel Briscoe, 1697), by the English architect and playwright John Vanbrugh (1664-1726):

Enter Seringe and Servant.

Serv. Here’s Mr. Seringe, Sir, was just going by the Door.
L. Fop. He’s the welcom’st Man alive.
Ser. Stand by, stand by, stand by. Pray Gentlemen stand by. Lord have Mercy upon us, did you never see a Man run through the Body before? Pray stand by.
L. Fop. Ah Mr. Seringe—I’m a dead Man.
Ser. A dead Man and I by—I shou’d laugh to see that, I gad.
Lov. Prithee don’t stand prating, but look upon his Wound.
Ser. Why, what if I won’t look upon his Wound this hour Sir?
Lov. Why then he’ll bleed to Death, Sir.
Ser. Why, then I’ll fetch him to Life again, Sir.
Lov. ’Slife he’s run through the Guts I tell thee.
Ser. Wou’d he were run through the Heart, I shou’d get the more Credit by his Cure. Now I hope you’re satisfy’d?—Come, now let me come at him; now let me come at him. Viewing his Wound.] Oons, what a gash is here?—Why, Sir, a Man may drive a Coach and six Horses into your Body.

The phrase originally occurred as to drive a coach and six horses through something. The earliest occurrences that I have found are as follows:

1-: From The State of the Protestants of Ireland under the late King James’s Government; in which their Carriage towards him is justified, and the absolute Necessity of their endeavouring to be freed from his Government, and of submitting to their present Majesties is demonstrated (London: Printed for Robert Clavell, 1691), by William King (1650-1729), Church of Ireland archbishop of Dublin:

The next Court for business (though not for Precedence) is the Exchequer: […] they thought themselves concerned to have an able Man, and one throughly Cordial to their Interest for the Chief Judg in it: for if he had wanted Sense or Law, though willing, as they found by Experience in some of the other Courts, he might have been unable to serve them in all Cases. They therefore fixed on Mr. Stephen Rice *, afterward Sir Stephen, who had formerly been noted for a Rook and Gamester at the Inns of Court. He was (to give him his due) a Man of the best Sense amongst them, well enough versed in the Law, but most signal for his inveteracy against the Protestant Interest and Settlement of Ireland; having been often heard to say, before he was a Judg, that he would drive a Coach and Six Horses through the Act of Settlement, upon which both depended.

* This refers to the Irish lawyer Stephen Rice (1637?-1715), Chief Baron of the Exchequer in Ireland.

2-: From Memoirs of the Most Material Transactions in England, for the Last Hundred Years, preceding the Revolution in 1688 (London: Printed for Tim. Goodwin, 1700), by James Welwood (1652-1727):

The next Court is that of Exchequer […]. It was thought fit, that one Rice, a profligate Fellow, and noted for nothing but Gaming, and a mortal Inveteracy against the Protestants, should fill the place of Lord Chief Baron. This man was often heard to say, before he came to be a Judge, That he would drive a Coach and Six Horses through the Act of Settlement.

The phrase then came to be used as to drive a coach and six through something. The earliest occurrences that I have found are as follows:

1-: From Henry Somerville, a Tale. By the author of Hartlebourn Castle (London: Printed for J. Bell, 1797)—as quoted in the Oxford English Dictionary (3^rd edition, September 2022):

There never was an act of parliament made but a good whip could drive a coach and six through it.

2-: From Confessions of a Politician, by ‘James Doubtful’, published in The Satirist, or, Monthly Meteor (London, England) of May 1810:

There never was an act of parliament, Mr. Satirist, through which a clever fellow may not drive a coach and six.

3-: From a declaration made on Tuesday 11^th June 1811 by Thomas Ainsworth, a Bolton manufacturer, to the House of Commons committee to whom petitions of Lancashire manufacturers, artisans, weavers, spinners, etc., were addressed—as published in Report on Petitions of several Weavers, &c., in Reports from Committees. Session 1 November 1810—24 July 1811:

“I have heard lawyers say that there is no Act of Parliament that they cannot drive a coach and six through.”

4-: From The Mirror of Fashion. In a series of Letters from a Gentleman of rank and taste, to a Lady of Quality. Letter X, about a royal mandate against extravagance in apparel, published in La Belle Assemblée; Being Bell’s Court and Fashionable Magazine (London, England) of April 1812:

It has been said in our times, “there is no act of parliament that a man may not drive a coach and six through!” and, in like manner, the beaux and belles of former ages found a path to conduct them to vanity fair.

5-: From Harrington, a Tale; and Ormond, a Tale (London: Printed for R. Hunter, and Baldwin, Cradock, and Joy, 1817), by the Anglo-Irish novelist Maria Edgeworth (1768-1849):

“My dear boy, your much-lamented friend and benefactor (is not that the style?) king Corny, who began, I think, by being, years ago, to your admiration, his own tailor, has ended, I fear, to your loss, by being his own lawyer; he has drawn his will so that any attorney could drive a coach and six through it—so ends ‘every man his own lawyer.’ Forgive me this laugh, Harry.”