Suhaib Majeed was convicted of conspiracy to murder and preparation of terrorist acts following a trial in the British capital.
His friend, Tarik Hassane, pleaded guilty to the same charges last month.
Majeed had got hold of a gun and ammunition and was discussing buying a moped when police arrested him, the court heard.
He had identified a police station and an army reserve barracks in west London as possible targets on the internet, prosecutors said.
Commander Dean Haydon, head of the Metropolitan Police's counter-terrorism command, told reporters officers suspected Hassane ‘was communicating with people in Syria’.
‘There is every possibility that he was in Syria but I can't confirm that,’ he added.
Jurors had been warned by the trial judge at the Old Bailey court to put events in Brussels out of their mind as they considered their verdict.