"We had to shut the main gates on Great Russell Street to prevent more people from coming in. It was the first time we did that since the Chartist riots of 1848 - although on that occasion the staff were actually on the roof, armed with stones."
Presumably that would be the occasion when, on 10 April 1848, tens of thousands of Chartists were barred from presenting their petition to Parliament, and consequently also missed visiting Trafalgar Square – still under construction at the time of the previous petition in 1842.
Finding themselves stuck on Kennington Common three years too early for the Great Exhibition, and some decades before the Northern Line was dug through, they were forced to march on the British Museum where they demanded novelty pencil sharpeners in the shape of the Rosetta Stone.
Furious at not having thought to open a tourist shop, let alone a cafe, the staff of the museum took to the roof where they fended off all further efforts to gain entrance.
NB. If you’re planning to quote any of this, I’d check one or two of the details – I’m working from memory.