Why Control and his team are singing Russian national anthem?

As explained here:

That’s when the show-stopper comes on, the office Santa (I’m fairly sure that’s a drunk Roy Bland (Ciarán Hinds), aka Soldier), slovenly dressed, adorned with a Lenin mask, and demanding his music, the State Anthem of the Soviet Union. The idea that the MI-6 office Christmas party would engage in a group singalong of their primary’s enemy’s national song (might this have come from somebody’s memoir? or from le Carré?) is just wickedly stinging, a cunning little piece of evidence of the service’s deep cynicism, the cynicism that both makes them highly effective at analysis and agent recruitment (nobody recruits an agent like the British, ask anybody in intelligence—they’re rivaled only by the Cubans, no lie) and renders them vulnerable to would-be penetrating moles of the right class and ilk.

It is a dark ironic joke.

The Circus job is (mostly) to oppose eastern-block spys who are controlled by the Soviet Union. Their enemy is the Soviets. But their enemy is also the group they most need to understand. They probably know the Soviets as well as they know their own society: they would not be effective spys if they were not.

They are also somewhat cynical. For example, in Smiley's interview with Karla (the Soviet spy boss) he says something like "we are not so different you and I" (shown in the TV series, recounted verbally by Smiley in the movie). This demonstrates a certain flexibility of perspective when thinking about their opponents.

So it is unsurprising that a bunch of somewhat cynical experts on the Soviets, would, ironically, sing their enemy's anthem at a fun event like a christmas party.

One suspects that Karla's team wouldn't be singing the British national anthem if they even had christmas parties. Karla is portrayed not as a pragmatic cynic but as a committed ideologue who would not appreciate the irony.