Back in the 60s, 70s, and early 80s there was more genuine difference between the political parties. Labour had a sizeable "hard left" membership that, while never gaining control of the party, could help shape policy. Even those from the left that took leadership roles in the party (Harold Wilson for example) seem to take a couple of step to the right once they had the position.
I suspect that part of the reason that in real life the left was broken was because of the press. Comments (in the popular media) that suggested that anyone with half a brain would emigrate if the Labour Party won didn't help.
This book imagines what might happen if a populist left wing MP (Harry Perkins) had become leader of the party, and then leader of the country. It also dreams that he doesn't make a "Wilsonian charge to the centre ground of British politics" after his election.
In response to his election as Prime Minister (the book starts as Perkins makes his way to London after the election), the Secret Service, the Press and the moderate elements of his own party (all of whom are accustomed to watching the left fizzle about like loons for a while, but never getting elected), decide to see if they can bring him down.
Some people have called this a fictional imagining of what might have happened to Labour politicians of yore. Others seem to see parallels to today's Labour Party. Maybe it has, maybe it hasn't got those parallels. I just call it a fun read. It rattles along quickly, is easy to get into, and is generally a fun read. Don't expect anything too deep and meaningful. If you go along for the ride, you'll enjoy it.