IIRC, Kingsley Amis' The Alteration is set in a 20th-century world where Arthur survived and had sons by Katherine. The Reformation's effects were quite limited: proto-Protestants (mostly English and Dutch) were exiled to the New World, where they flourished and led in techology (and also had Native American slaves). Martin Luther never led the Reformation: he became Pope and instituted what we would consider a counter-Reformation. Henry is alluded in passing as a very nasty piece of work. In The Alteration's 20th century, science is associated with magic, which is associated with evil.  And most germane for the plot, castrati are still in use, as women are not allowed to sing or act in theatres. The (alternate) historiography of The Alteration was quite Whiggish and anti-Catholic, I thought, but it was still a good novel.

In either of your scenarios, Jenny, everything I think would have been very altered. For instance, would the Spanish Armada have been defeated-would Spain rule England? England might continue being Catholic, obviously the famous six wives would not have existed, and if Mary Boleyn's children were Henry VIII's (hmm, I wonder if they were? I ponder that constantly!) they would not have existed, obviously as would Elizabeth, Edward, and Mary Tudor, etc…Wow, interesting to think about!

How far back do you want to go?

If William the Bastard had lost?

If Richard the Lionheart   had never gone on the Crusades?

If Edward II had rallied enough troops to defeat Isabella and Mortimer?

MY personal fave…If Richard III had won the Battle of Bosworth?

” Fatherland” is a very good novel. I read it in a single 5 hr stretch.