This rather unusual pump is in Montagu Square, Marylebone. (It's not unique: there's a very similar one in neighbouring Bryanston Square.) The surrounding houses were built in 1811, although the private garden in the centre is probably a few years older.
Dating from the early nineteenth century, the pump is a little
mysterious: unlike most Victorian cast iron, it doesn't bear a foundry
name. Presumably the Doric column is intended to reflect the elegance of the Regency houses surrounding the garden, whose inhabitants used the pump. Unusually, the square remains residential today - although the residents enjoy rather more modern plumbing!