Taylor was spotted filming a new video in London Saturday night, where she shot around a few different sets "Green Eggs and Ham" style ... on a double-decker bus, in a cab and on a bike atop the Millennium Bridge.

She's rumored to have been spending time in London recently -- even reportedly hosting a secret listening session for fans there to hear her new album, "Reputation," over the weekend.

Looks like the city's grown on her enough to wanna shoot there ... but not enough to be fully out in the open. You see her trying to avoid cameras with her crew ... and on the move.

Until the next T-Swift sighting ...