The Romans of course, one of the most successful empires known to man and its capabilities would have only reached further across the world had Caesar not been assassinated.
Julius Caesar was calming down with his conquests. After seizing the empire for himself, and becoming dictator, he had already captured substantial amounts of land (rich in resources), in Gaul, Iberia, and Gibraltar. He even took lands in Syria, Aegyptus, and Cyrenaica. He hadn't the need to expand further. The richest of lands in the known world were already Roman, thanks to him.
However, if I truly had to pick Romans or Britons... It'd be very hard to decide, but probably ultimately come down to the Romans. The Britons were master ambushers and masters of fear. The Romans were master soldiers and masters of discipline. Although the Britons had ambushed the Romans countless times, they almost always lost due to the Romans knowing how to keep cohesion and rotating tired soldiers. It kept fresh troops at the front the whole time, and often times, they fought exhausted British warriors.