Worth reading. The novel is extremely well written.
The two divergence points are 1) Henry VIII's older brother had a child with Catharine of Aragon - which created a Catholic heir. Once Henry VIII rebelled against this and attempted to seize the throne, he was defeated by a combined Catholic force of other nations; 2) Martin Luther making his own deal with the Papacy, choosing to reform it from within by becoming Pope and introducing a system of rotating the Papacy among all nations, creating allegiances among nations that felt they were excluded by exclusive Italian choices prior to this.
In present day (well, 1976) England of this TL, Europe is much more united with an active Papacy with temporal authority over the Italian states and dominion over all forms of information (the two major daily newspapers are controlled by the Catholic Church of the Papacy, and by the local Catholic Church organs of England), which suppresses technology and all forms of innovation that is not crucial to their authority.
Shakespeare (look, this is an English novel about England by an English writer, of course there is going to be a reference to Shakespeare) was driven out of Catholic Europe in this TL, his house burned to the ground, his works property of the Church that "adjoined" them. Kyd's "Hamlet," however, survives.
The united Catholic Europe is counterweighed by a united Ottoman Empire in the east, creating a sorta Cold War between the two nations that will grow hot soon due to machinations of an English Pope, who as stated before is clearly Harold Wilson. I mean, so clear, that even I - an American with only a vague knowledge of '70s British politics - got it was Harold Wilson.
Other special guest appearances include hard left Labour MPs as the Secular Arm of the Catholic Church that does all their dirty work. When an execution is to be done, the Church turns over the guilty to the Secular Arm. They function like KGB-Gestapo in this world.
And there is special cameo by Count Paolo Maserati (yes, as in the car manufacturer Maserati, but there were no Paolos in their family at the time the book was written). Maserati is the Inventor General of the Catholic Church and conducts secret chemical experiments on the prison populations of the Church controlled gulags sprinkled throughout Europe. Italy alone houses 150,000 prisoners apparently. Maserati does these experiments at the behest of the Church to find a way to limit the burgeoning population of Europe.
The inclusion of Maserati of all people here is an author on board moment. Amis uses the book to score political points against the left wing of his own country, his interpretation of what makes art good (sexuality), and his latter life rabid anti-Italian prejudice. Per his own son, Amis was convinced that Italian culture was taking over England in the '70s and saw pizza parlors as an insidious destruction of English culture and cuisine and was capable of ranting on the subject for hours.
In the novel, The Pope is not pleased by Maserati's efforts, and judges them to be ineffective or population draining due to mutations they'd cause in the general population. So, instead he schemes a European wide war with the Ottoman Empire that would allow Ottomans to invade most of Europe and loot and kill as many as they can, until a counter attack would destroy them. This goes off without a hitch in the book's epilogue.
These are however the sidebars, the main story of the book is the efforts of a 10 year old English boy to escape castration, that is planned for him to preserve his singing voice. His adventures and contemplations through this alternate England are described in a fine literate manner that I wish I'd find more in current crop of alternate history.
Amis creates a world, and populates it with real people. There is even a subdued attempt to change the way people talk in this TL. Nothing so idiotic as phonetic pronounciation and broad attempts at accents, but the use of a couple archaic words, the odd ways characters begin sentences, the overuse of certain words by people in positions of authority, and the flowery way bureaucrats discuss things creates a clear sense of style of this world.
Amis is a master of the English language. And this is not just a good AH novel, it is a good novel - period.