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A friend just send me this link 9assumingly because it was filmed near the Criterion?) with the request to organise flashmob of Sherlocks..
[video] [/video]
The questions would be, where? St. Bart;s seems a bit morbid - what about running up the stairs at the BM / British Museum?
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If they want to refer to canon then Covent Garden might be an option. Outside and on the steps at The British Museum would be good too. The Criterion Restaurant or the Cafe Royal are both options too. No point doing these things if no one sees them!