Heh! When my daughters were teenagers, we used to commit "Drive By Singings"

Windows rolled fully down and top of the lungs singing at people we passed. We sang everywhere, under shelter at Disneyworld during thunderstorm, shopping in KMart, strolling around Charlotteville's downtown mall. We got strange looks. Mostly we got smiles. We got people taking pictures. Didn't matter. We were singing for us.

I think we all should just sing where ever we are. Standing in queue at hole in the wall bank machine, during weekly shop at (pick your favourite supermarket), on the bus/tube to and from work, at the parking attendant putting ticket on your window screen. Sing rousing songs with easy refrains. Don't be shy. Encourage people to join in. Not only in London, but everywhere.

Stand up Catters and sing.