A lovely day ... I presume its origins (in the English Catholic propers for the Blessed Martyrs of England and Wales) lie in the fact that on May 4 1535, the primitiae of the English Martyrs, the Carthusians of the London Charterhouse, died at Tyburn.
Originally, today's commemoration referred only to Blessed Martyrs. Of course, the Carthusians were among the sixty three who were beatified equipollently by Leo XIII (29 xii 1886 and 13 v 1895). They were then canonised on October 25 1970 by S Paul VI. (I think 'equipollently' means that, since the Holy See had tolerated the set of portraits of these martyrs, with symbols of Holy Martyrdom, being venerated in the English College in Rome ...)
I don't detect a problem as between sancti and beati. Sharp eyed readers will have noticed that, in the Authentic Use of the Roman Rite, beati is (invariably?) used in the collects, irrespective of status. The custom has survived since the time before formal beatification introduced a distinction. But as far as labelling does matter, I suppose we should now talk about them as the Holy and Blessed Martyrs of England and Wales ... or simply as the the English and Welsh Martyrs.
Rank of this Festival? The current English RC Calendar, and the Ordinariate Calendar, call this Commemoration a Festum. In pre-decimal currency, that would probably mean Greater Double or possibly a Double of the Second Class. (It was a Greater Double before the Great Confusions; but the subsequent canonisations, and the greater number of commemorands, may suggest promotion.)
English Anglicans observe, on May 4, "English Saints and Martyrs of the Reformation Era". And, in Oxford's University Church, Catholics and Protestants are listed promiscuously together. I know people, dear things, mean well, but I don't like it. I prefer the wit involve in mixing up the relics of S Frideswide with the remains of a Protestant lady ... proclaiming that here are buried both superstition and true religion ... and leaving us all free to reduce the formula to our own precisions. Far more Oxonian.
Some day, I must tot up how many of these Martyrs had Oxonian connexions.