Barcode Miniature Sheet Covers
This is a Royal Mail marketing initiative, introduced in 2014. Whenever Royal Mail issue a new miniature sheet they also issue a version with a barcode on it (featured down the left hand side).
For example, the First Day Cover shown here on the right features the barcode miniature sheet that was part of the Christmas issue (3rd November, 2015). With the extra barcode down the left hand side of the minature sheet, they are often more interesting than the normal stamp issue.