Thetford is a market town and civil parish in the Breckland district of Norfolk, England. It is on the A11 road between Norwich and London, just south of Thetford Forest. Thetford is traditionally thought of as the royal residence of Boudica Queen of the Iceni. ~Wikipedia
The building being used as the Post Office in this postcard is now a shop. The current Post Office is now next door; towards the church.