Contrary to what is shown in the series Robert Rogers was not Scottish. Though he did lead troops for the British during the Revolutionary War Rogers was originally born in Massachusetts.
The British flag that is used in promotional materials is the modern day union flag and includes the cross of St. Patrick (representing Northern Ireland) on it. The cross of St. Patrick was not added to the flag until 1801. Flags within the series itself are, however, lacking this anachronism.