London's top police officer, Commissioner Bernard Hogan-Howe, gave a lesson on the dangers of knife crime to teenagers at Edmonton County School - including the friends of a pupil who was killed last year.
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A lamp in the tower which houses Big Ben is to be switched off for the first time in more than 70 years.
Sir Bruce entertained audiences for more than seven decades.
Work in the tower will also pause over the bank holiday weekend.