Firstly, the britons at this stage of history were never a huge force to be reconned with, Julius Caesar wasn't defeated but left because he couldn't stay over the winter (lack of supplies, and probably not wanting to be out of touch with Rome for 6 or more months).

As for the gallic tribe in englnd this is done so that the gauls have more staying power and can survive further into the game, this will likely be changed for RS2.