Bit of a riff on James Bond....make sure you read the comments for some of the choices.

https://www.hodinkee.com/articles/what- ... james-bond

My guess is Rolex Oyster Perpetual, or a Smith's, given the era. Patek, the author's choice, wouldn't be appropriate

What do you think spies are: priests, saints, and martyrs? They’re a squalid procession of vain fools, traitors too, yes; pansies, sadists, and drunkards, people who play cowboys and Indians to brighten their rotten lives. Do you think they sit like monks in London balancing the rights and wrongs? I’d have killed Mundt if I could, I hate his guts; but not now. It so happens that they need him. They need him so that the great moronic mass that you admire can sleep soundly in their beds at night. They need him for the safety of ordinary, crummy people like you and me.

--John le Carre, The Spy Who Came In From the Cold.

I agree with JAS; if Smiley were to have worn any watch outside of something totally mundane, le Carre (who, sadly, passed away last December) would have mentioned it. The fact that he never did speaks volumes.

Cartier Tank or a Longines three-hander.

Haven’t read the books, only watched the TV series with Sir Alec Guinness, and the „Tinker Tailor Soldier Spy” film with Gary Oldman as Smiley.
Going by how Smiley looked in all of the above, he didn’t seem like someone who would be willing to buy some fancy watch. I’d say it would be more likely that he’d choose anything for the sake of having a watch without caring much about it.
Besides, he used to be an undercover operative, so he’d probably have the habit of choosing items that blend in rather than stand out, and are of the more disposable kind.
That’d suggest some brand popular in Britain back then - a house brand like Rotary, a popular jewelry store label like Bravingtons. Or a Smiths.
Basically, the dirt-common thing. However, the sort of brand that’d have been available everywhere - like Tissot - would also make sense.

Interestingly, Ian Fleming was an author who name-dropped every lux brand that he could--which feeds well into the whole, tiresome what-fictional-character-wore-what-watch debate that has been going on for about three thousand years in WIS circles. Ian Fleming, in one of the Bond books, stated that Bond wore a Rolex. This was enough to launch a million fucking threads about what James Bond would wear with his Argyle socks.

John le Carre, who was an infinitely better writer than Fleming, never stated what watch George Smiley was wearing. This goes with the very fine writer Elmore Leonard, who, in something of a master class, told aspiring writers to never describe the physical look of his characters, unless it was important to the story, which it really never is.

So, what would George Smiley have worn in his wrist? We know, from the books, that he used a cigarette lighter, to light Karla's smoke in India when he tried to turn him--and that lighter was given to him by his adulterous wife, Ann. The lighter, which Karla nicks, returns at the end of Smiley's People, when Karla defects to the West and hands the lighter back to Smiley, who is there to take him in from the cold.

From this, we would expect that Smiley was wearing a watch given to him from Ann. And as the lighter was obviously not a standard Zippo, the watch could have been anything.