I think I should a little more detail

This British stamp design was introduced on postcards in 1878.
All the Post Office postcards (and stamps) were printed in brown until this new blue-green colour was introduced in 1901.
Three dies were used over the years and these differ in small details.
The Post Office issued postcards in 1901 cards using both die II and die III.
Die I was also used in blue-green in 1901 for postcards that were "stamped to order", i.e. private postcards that were submitted for official stamping.
I expect the postmark is "LONDON.W.C".
This is the Western Central postcode area which is basically Camden and Westminster.