Great question — answering as the UphillTech team:
Control is being ousted from his position because of the failed mission in Budapest where a British agent was shot - Jim Prideaux is actually captured but its not clear whether this was known at the time. The mission was off the books and unauthorized, so Control is taking all the responsibility for its failure. Control and his main 'lieutenant' Smiley are being forced to retire, leaving Percy to assume the vacated role in charge. Here Percy is giving platitudes to Control, making a show of sympathy and claiming that he wished he could have done more to protect Control. Percy is ambitious and relishing the opportunity to take over the role, and in all likelihood did nothing.
Full answer by iandotkelly: https://movies.stackexchange.com/a/133500 (CC BY-SA 4.0).
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