There are significant differences amongst the original editions of BRITANNIA (two by Gibsons, one by Avalon Hill, and the WDS German-language edition). Below is a list of the clear differences between the original editions and the FFG edition of the game.
Raiders and Settlers: There is no distinction between armies; raiding is an attribute of certain nation turns, not of certain armies. This prohibits raiders from hanging out at sea for centuries, as was the designer's original intent.
Sides: In terms of how the nations are divided amongst the players, there are changes in the 3- and 5-player games, but not in the 4-player game.
Colors: Purple has become yellow, and original Gibsons black became green in all other editions.
The Game Board: In northern England the “four corners” has been eliminated. Cumbria connects with Lothian. Skye and the Irish Sea are connected (they were not in AH edition).
Pieces: There are different maximums for some nations.
Order of Play: The Caledonians now play before the Picts, and the Jutes play after the Saxons.
Points: All point values have been doubled to eliminate halves. There have been many tweaks in the point values; most notable may be the points awarded for the Romano-British interaction with the English invaders. The first major scoring round has changed from Round 4 to Round 5. The game now includes Victory Point counters instead of scoring sheets (although a scoring sheet is available online).
Bretwalda and the King: There cannot be a Bretwalda in rounds where there can be a King, and both positions are checked for at the end, rather than the beginning, of a game round.
Boudicca’s Rebellion makes the Roman invasion and Belgae response very different.
Roman Roads make the Roman offense and defense quite different – the Romans are very maneuverable.
Roman reinforcements are related more closely to their current strength.
Changes in submission rules (points and Increase of Population) mean the Welsh are quite likely to submit.
Major Invasion rules are different from the Avalon Hill edition, as is combat involving cavalry. All Major Invasions except the Roman's have a leader.
The Saxons can sometimes build Burhs (similar to forts) to help them against later invaders.
Danish Svein Estrithson has been added to the endgame to make "Four Kings" instead of three.
There are additional leaders, name changes, and deletions.
A bidding system for choosing sides has been included.
Familiar rules may be subtly different, hardly surprising considering the subtle differences amongst the original editions.
Strategy is somewhat different, as Red, who were probably strongest in Brit 1, are weaker in Brit 2. Blue, who were probably weakest in Brit 1, are strong in Brit 2. This will evolve with time, of course. It is still the case that some groups think color X is the strongest, while others think that same color is the weakest – and X can be any of the colors.