[In England] after 1170, … as part of the Compromise of Avranches, Henry … agreed that the secular courts, with few exceptions (high treason being one of them), had no jurisdiction over the clergy. … Defendants demonstrated their clerical status by reading from the Bible. This opened the door to literate lay defendants’ also claiming the benefit of clergy. In 1351 … the benefit of clergy was officially extended to all who could read. …

The Biblical passage traditionally used … [was the first verse of] Psalm 51 [which] …. became known as the neck verse, because knowing it could save one’s neck by transferring one’s case from a secular court, where hanging was a likely sentence, to an ecclesiastical court, … [where] if the defendant swore an oath to his own innocence and found twelve compurgators to swear likewise … he was acquitted. … By the 15th century, most convictions in these courts led to a sentence of penance. …

Henry VII decreed that non-clergymen should be allowed to plead the benefit of clergy only once … [and] were branded on the thumb, and the brand disqualified them from pleading the benefit of clergy in the future. (In 1547, the privilege of claiming benefit of clergy more than once was extended to peers [i.e., Nobleman] of the realm, even illiterate ones.)

In 1512, Henry VIII further restricted the benefit of clergy by making certain offences “unclergyable” offenses; … This restriction was condemned by Pope Leo X … [and led] to Henry VIII splitting the Church of England from the Roman Catholic Church in 1532.  In 1575, a statute of Elizabeth I … the benefit of clergy … it did not nullify the conviction, but rather changed the sentence for first-time offenders from probable hanging to branding and up to a year’s incarceration.

More here.  The English literate classes had quite a conspiracy going to help themselves at the expense of others!  HT Greg Clark.

Added 11a: From stats Clark showed in a talk, it seems most folks people did not invoke this benefit.  This was not a benefit given to all.