On 12 April 1606, a new flag to represent this regal union between England and Scotland was specified in a royal decree.
But The Union is still young in 1755. The Battle of Culloden in 1746 proves that.
Although both shared the same King since 1606, it isn't until almost 100 years after that Scotland and England Parliaments unite in 1707.
The flag below is the one we think flew on land. Notice the diagonals are missing because at the time Ireland was not united with Scotland, England, Wales.