In Tinker Tailor Soldier Spy (2011), Polyakov is taking information from the mole and passing it to the Soviets. So why didn't Smiley arrest Polyakov?
Polyakov is a diplomat, with a role as a cultural attaché. As such he has diplomatic immunity.
Arresting a diplomat is an incredible breach of international norms and would put your own diplomats worldwide at risk. Article 29 of the Vienna Convention on Diplomatic Relations states:
The person of a diplomatic agent shall be inviolable. He shall not be liable to any form of arrest or detention. The receiving State shall treat him with due respect and shall take all appropriate steps to prevent any attack on his person, freedom or dignity.
Theoretically the UK could order Moscow to withdraw Polyakov from the country, but even that is a sanction reserved for very serious breakdown in relations between countries.
Expelling diplomats does happen, but it's a decision made at a political level in a country, not something that an agent like Smiley can do.
It's known that some diplomats are spies for their country, and the way you deal with that is by having counter espionage operations against them, which is what we are seeing in the movie.
It's also suggested that the Russians would recall Polyakov to Russia and probably kill or imprison him, due to his failure. May 23, 2022 at 0:24
@magarnicle .... Well that's from Percy Alleline's perspective where Polyakov is their source called Witchcraft and is giving them secrets. They's why they stop Connie Sachs investigating him. However although Polyakov is giving them some stuff (of unknown quality, to appear genuine) his primary purpose in London is running Bill Haydon as a double agent.– iandotkelly ♦May 23, 2022 at 12:06
FYI, when a government expels a foreign spy, especially one who was operating under "official cover" (i.e. under the guise of being a diplomat), the host country declares them "persona non grata," which is a designation provided for by the VCDR. Importantly, the host country is legally entitled to apply this designation to anyone for any reason, even before that person enters the country, and without having to explain its decision, which is essential when the reason is espionage (which the foreign country will inevitably deny to the end).– TomMay 24, 2022 at 1:29
Interesting, but I was mainly referring to Polyakov telling Haydon at the end: "Why don't you cheer up? You're probably going to get a medal and a flat in Moscow. I'm the one that's going to get sent to Siberia." May 24, 2022 at 6:23