The whys and wherefores of what's happening in this novel are its weakest element. The finale doesn't clear things up, and, if anything, it just muddies the water. Is the Earth the bad guy? Does Mandrake have some strange psychic hold over England? It's all a bit sketchy. In this regard, and in other respects, Mandrake feels very much like a Dr Who or Quatermass story, and certainly Queston's name is almost a joking amalgam of those two characters names. Queston even gets a companion, Beth, who also becomes his love interest. Queston actually does very little in the novel except witness events, but he's a stalwart and agreeable character.
What the novel lacks in coherence, it makes up for in atmosphere. Combining elements of 1984 and tropes from John Wyndham's SF novels, and with an added hint of folk horror, Cooper creates a dread-soaked and demented England that's been brainwashed and bullied into believing happiness only comes from insularity and xenophobia. Here's a minor character's view of the changes:
"I never voted for this government, I'll tell you that, but they've done well. Could have pranged everything, but they've given us peace and quiet. Keep ourselves to ourselves, that's all we've ever wanted. All that Common Market nonsense--England's an island, isn't it?"
A political leader persuades people to take a self-evidently disastrous course of action, like, say, Brexit? Pure science fiction. This was Cooper's first novel, and while its internal logic is flawed, the world-building and atmosphere are excellent. Not a must-read, but a fascinating and well-written curiosity.