Think it's intended purely to improve the grip and solidify the toy as a bluff. Prideaux's come unarmed on the mission (unlike the book) and improvises resourcefully, if pre-Macgyver ... a bit lamely. Disarming the taxi driver by hand, putting that real gun down and then threatening him to exit with the toy placed against his neck does take some chutzpah!
What's interesting is the taxi driver dressed as, and believed to be Prideaux is shot dead immediately - not resisting in any way. It looks decidedly premeditated and that, initially, Karla never had any intention to spare him as agreed with Haydon.
Prideaux's conversation with Smiley at the school had Prideaux relating about how interested Karla was in Control's conjectures towards the mole's identity. I got the impression Karla wanted to know how close Control was to unmasking his source, so I would think Karla would have wanted Prideaux alive.
Jim's surrender of Control's code under torture is, at that point, inconsequential for Karla. Control has been deposed and discredited with no way back, a beaten docket - vanquished through the coup engineered out of the shambles of Operation Testify. Telling that Karla's reaction to what should have been a triumph was an underwhelming couple of seconds thought about the code and offering Prideaux a cigarette. The discursive two-way nature of the interrogation confirmed Prideaux's suspicion of a high up mole both from the quality of tittle-tattle and detailed knowledge of Control's service flat. You'd have to assume Karla's loquacity was very much intended as a prelude to Prideaux's imminent mortality in the ruthless manner of Irina, Boris and Lvlov to ensure the knowledge would die with him. Reading between the lines only Haydon's strenuous objection saved Jim.
I should say it's likely Karla already had the Tinker Tailor code from Haydon's contact with Prideaux before the mission (though there's more support in the book than the TV version for this).