As I explain in my book Edward VI in a Nutshell:
Centered in Devon and Cornwall, the Prayer Book Rebellion was spurred by the Catholic and West Country dislike of the new Anglican Book of Common Prayer they were forced to adopt. Archbishop Cranmer’s liturgical writings are considered masterworks from a modern perspective, but they were too different from the accustomed Catholic mass to do anything other than antagonize adherents of the older belief system. Rather than stamp out Catholic practices, the forcible application of a Protestant prayer book merely made the Catholics more resolute in the face of oppression. Nonetheless, the rebels didn’t demand a strict return to Catholicism, claiming that they would be satisfied with the Anglican prayer service used during the last decade of Henry VIII’s reign.
There was more to the Rebellion than the Prayer Book, however. Edward’s uncle, Edward Seymour, the Duke of Somerset and de facto king in his position as Protector of the Realm, knew that:
the uprisings were as much over land enclosures and inflation of food prices as they were over Cranmer’s new prayer book. To his credit, he even understood the injustice of the gentry seizing and privatizing lands that were traditionally for common use (Loach,1999). It was in sympathy for the rebel’s physical needs that the Protector issued a proclamation on 2 July in which he promised to regulate the price of food, among other things (Skidmore, 2009). Unfortunately, what we now see as compassion for the poor is what the Privy Council saw as treasonous support for those who would overthrow the ‘natural’ order and have the base rule the elite. One of the ‘crimes’ Somerset was later accused of when he was removed from the office of Lord Protector was that he, “had not taken care to suppress the late insurrections, but had justified and encouraged them” (Burnet, 1848). It was only at the urging of several council members, including his ally and close friend Archbishop Cranmer (who was personally incensed by the burning of prayer books and the rejection of English authority), that Somerset took a hard line against the rebels. He dispatched German and Italian mercenaries into the West under the command of Baron John Russell, the High Steward of Cornwall. This worked out about as well as you would suspect.
The combined forces of the English troops and the mercenaries, including Italian arquebusiers and the German Landsknechte, slaughtered the Rebels at Clyst St. Mary on 5 August, 1549. Worse still, nearly a thousand English prisoners had their throats slit that night by Russell’s orders when the army camped at Clyst Heath. The surviving Cornishmen retaliated at Clyst Heath the following day, but they were again decimated by Russell’s mercenaries. Somerset and other firm supporters of the Reformation continued the punitive measures against the West long after there was any organized rebellion left. For his services, Baron Russell was made the Earl of Bedford in 1550, and created the Lord Lieutenant of Devon in 1552.
Unwisely undaunted, rebel leader Humphrey Arundell regrouped the dissidents at Sampford Courtenay in Devon and prepared to fight again. Alas, what he didn’t know was that he had been infilrated by a traitor. Arundell’s own personal secretary, John Kessell, had been relaying Arundell’s every move and plan to Baron Russell from the get go. Russell, whose forces now had the additon of more troops under Provost Marshal Sir Anthony Kingston, was in command of an army of more than 8,000 men, and he moved into position to eradicate the remaining rebels.
Heavy divisions led by Lord Grey and Sir William Herbert stormed the rebel encampment, while Russell himself would follow behind. This was not as simple as Russell had envisaged: the rebel camp being more strongly manned than he had thought. A vicious gun battle, lasting roughly an hour, gave time for Russell’s two other divisions to make their move … With almost the entire government force ranged against them, the rebels withdrew into the village where they came under heavy bombardment … the battle might have been won for the Cornish and West Devonians had they possessed any cavalry … Contemporary Exeter historian John Hooker wrote that the rebel army would not surrender until most of their number had been slain or captured. Lord John Russell was quoted that his army had killed between five and six hundred enemy and his pursuit of the rebel retreat killed a further seven hundred
The surviving rebels fled to Somerset but “they were caught and mostly hanged, drawn and quartered by troops led by Sir Peter Carew and Sir Hugh Paulet.” Carew and Paulet also murdered any man who ‘might’ have been a rebel, so that eventually more than 5500 men were slain in Cornwall and Devon. Humphry Arundell was captured and taken to the Tower of London before being executed a few months later. Only a tiny fraction of the original rebel forces were able to escape back south of the River Tamar.
A final effect of the Prayer Book Rebellion was the death of the Cornish language. Before to the rebellion, the Cornish language was growing and being used as the primary language of the region, and Cornwall had forced England into several political compromises treating the the area as another “nation”, much like Wales. After the rebellion, England forbade the translation of the Prayer Book or Bible into Cornish, unlike Welsh, which was preserved in large part due to the Biblical text and liturgy.