The Queen has been helping the National Theatre celebrate its 50th anniversary. The theatre opened its doors in 1963 with the Hollywood star Sir Laurence Olivier as its first director and over the decades has staged 800 productions.
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The 24-year-old was unarmed and the incident is not being treated as terror-related.
Some platforms have been taken out of service with disruption expected to continue until the end of the day.
The Met Office has extended its weather warnings for snow and ice to cover vast swathes of the country, including London and the South East.