this model is a 1953 so the rear hub tells me
If your bike does have a Sturmey Archer three speed hub you may be able to date the bike by the markings on the hub.
Hubs after World War 2 From 1947 to 2000 Sturmey Archer clearly marked their hubs with a date. This hub date marking will show the last two figures of the year and the month.
The article goes on to explain how to date older SA hubs.
Here is the 1950 Armstrong catalog.
This picture looked to me like a good match
Three speeds and dynamo hubs were offered as an option