The following flags represent Great Britain or one of its predecessors. These flags can be used as is or as inspiration.
OTL British Flags
The Kings Colors, or Great Union Flag, with only the flag of England and Scotland used. Used 1606-1707
The White Ensign, flag of the Royal Navy
The Red Ensign, flag of the British Merchant Marine. Often defaced for use by colonies/nations, namely Canada
The Blue Ensign, the flag of the British government. Often defaced for use by colonies/nations, namely Australia and New Zealand
Subdivisions of the UK
British Colonies/Commonwealth Nations
British Antarctic Territory
British Guiana, 1875-1906
British Guiana, 1954-1966
The Union flag as pictured by James Gillray
The flag of the Ultranationalist English Homeland Party
The Flag of England known as "The Cross of Life" that was used during World War 1.
Union of United Kingdom
Other UK Unions