From what I understand, the eagerness of the British to cave into Stalin's demands lay in fears of repercussions toward those British POWs who had been liberated by the Soviets.
However, beyond the obvious answers of Stalin's paranoia and a desire for revenge, what drove Stalin to take such a hard stance on the issue? Is any primary source material available to back it up or does this episode remain in the realms of speculation?
Because Russian prisoners who refuse reparation to the USSR are very likely to be anti-Communist or at least anti-Moscow. Allowing them to live abroad strengthens the Russian emigre anti-Communist movement which had its start with with white emigres after the civil war. During the interbellum era Soviet secret police fought a shadowy war against those movements as they were attempting to destabilize Communist rule in Russia itself.
Not only that but many (but by no means all) of the prisoners had supported the Nazis in various ways, the Nazis had formed several Russian units from PoWs as an anti-Communist Russian "liberation" army. In that sense some of those prisoners had committed treason and the Soviet government wanted them brought back to the USSR for trial and execution.