From Stephanie Mann at Catholic Answers:
The first martyrs were hanged, drawn, and quartered during the reign of Henry VIII; the last martyrs were executed during a wave of anti-Catholic hysteria during the reign of Charles II. They were accused under different laws and for different reasons: for refusing to swear to the spiritual authority of the monarch, being priests in England when it was an act of treason, aiding and abetting priests, attending Mass, celebrating Mass, or all manner of other grave accusations.
Their sufferings and deaths were known in the Catholic community at the time: Reginald Cardinal Pole, the son of a beatified martyr (Margaret Pole), expressed his horror at the martyrdoms of Thomas More; John Fisher; and the first martyrs in this group, the Carthusians John Houghton, Augustine Webster, and Robert Lawrence. Saint Philip Neri hailed the missionary priests leaving the Venerable English College in Rome by saying, “Salvete flores martyrum” (Hail! flowers of the martyrs) in the 1580s as depictions of the martyrs’ sufferings decorated the walls of the chapel in that college. One of the last vicars apostolic of the Penal era, Bishop Richard Challoner, collected the stories of the martyrs in 1741 in Memoirs of Missionary Priests and other Catholicks of both Sexes who suffered Death or Imprisonment in England on account of their Religion, from the year 1577 till the end of the reign of Charles II.
None of the martyrs of the English Reformation era—not even Thomas More and John Fisher—was even beatified until late in the nineteenth century. The first cause did not begin until 1874, almost a quarter-century after the hierarchy was re-established in England by Pope Pius IX. His successor Pope Leo XIII beatified fifty-four in 1886 and nine more in 1895. Pope Pius XI beatified 136 more in 1929 and canonized Fisher and More on May 19, 1935.
The selection of the Forty Martyrs was presented in 1960 and approved in 1961: they were chosen on the basis of their popularity and the devotion shown to them in England and Wales. Miracles attributed to their intercession were investigated and documented (Pius XI had canonized More and Fisher equipollently without verification of medical miracles); their canonization was announced by Pope Paul VI and approved by the hierarchy present at the consistory of May 18, 1970.
There was one delicate issue: the relationship between the Catholic Church and the Church of England. Representatives had met in Malta and organized the Anglican-Roman Catholic International Commission (ARCIC) for ecumenical discussions. Unitatis Redintegratio, the Decree on Ecumenism of the Second Vatican Council, had singled out the Church of England (“Among those in which Catholic traditions and institutions in part continue to exist, the Anglican Communion occupies a special place,” Chapter III, paragraph 13). Michael Ramsey, the archbishop of Canterbury, and Pope Paul VI had met several times, and the pope in 1966 had given Ramsey a ring that he was wearing—an extraordinary gesture. (Read more.)