In 1707 the union between England and Scotland was formalized as the United Kingdom of Great Britain by Queen Anne.
According to historians, one of the queen's decrees was that the Union Flag Red Ensign, the flag consisting of a solid red field with the Union Flag in the canton, now called the British Red Ensign, was to be used as the official flag for the British Navy and the commercial shippers.
And it is this flag that was the official flag for the use of the British Army, thereby being flown from every fortification in North America from 1707 to 1775.
The text of the First Article of the Queen's 1707 Union With England Act, in which the official flag of Great Britain is discussed follows:
"That the Two Kingdoms of Scotland and England shall upon the first day of May next ensuing the date hereof, and for ever after, be United into One Kingdom by the Name of GREAT BRITAIN;
And that the Ensigns Armorial of the said United Kingdom be such as Her Majesty shall appoint
[My note: in this case mostly red...the white and blue ensigns were formerly used to differentiate ships and fleets]
and the Crosses of St Andrew and St George be conjoined in such manner as Her Majesty shall think fit, and used in all Flags, Banners, Standards and Ensigns, both at Sea and Land."
This link discusses a Red Ensign allegedly found at Fort Bedford PA:
"Whether the Fort Bedford Flag was ever actually flown at Fort Bedford cannot be known because of a scarcity of records on the matter. According to popular tradition, the flag was removed from Fort Bedford by a group of young men shortly after the Declaration of Independence was signed."
That link discusses the provenance, chain of ownership of that found Red Ensign.