10 Downing Street is the headquarters of the Government of the United Kingdom and the official residence of the Prime Minister. (It is actually officially designated as the residence of the First Lord of the Treasury, an office which has effectively been merged with that of Prime Minister since the beginning of Britain's modern parliamentary system.)
Alistair Walsh and the major were scouting Downing Street in order to plan an effective attack on 10 Downing Street on the day that Walsh was arrested. Walsh had reflected that the major was a good deal more conspicuous than would have been ideal for the assignment. However, it was Walsh who was arrested.