Originally posted by: sparkyclarky
An entertaining thought, but almost impossible to accurately spec out. The Romans were the second most effective military force in world history (Alexander the Great ranks first, due to sheer area conquered in a short time span). They proved themselves militarily superior to all of the people throughout the Mediterranean, Northern Africa, and Gaul. Only after a long drawn out decline, did their military superiority finally dim. This superiority was the result of very advanced battlefield tactics (partially drawn from the Greek phalanx), combined with outstanding military leadership (e.g. Caesar, Scipio Africanus, etc.). The British are an entirely different beast. Much of their military supremacy was a result of their incredible naval prowess (not the Romans biggest strongpoint - but not too weak either (Actium)).
The British land tactics weren't drastically superior to others during their time period - one of the reasons that they were unable to form an empire the size of the Roman empire (whether they wanted this burden is another question altogether). Yes, gunpowder would give the British some advantage. However, as others here have brought up, it cannot be forgotten that the Romans had their own set of ranged weapons, including archers and bolts. Trained archers could do quite a number on the British ranks, especially considering that the British have little to no protection from piercing weapons (which the ancients did have). The Romans would also be more effective melee fighters, because their primary forces are designed to accomplish this task. Thus, instead of having an all purpose unit that isn't spectacular at any one area (British), the Romans would have multiple independent units with individual areas of expertise. This combination of units could quite easily prove the decisive factor in a Roman victory, assuming that the initial shock of gun powder doesn't completely cause chaos within the ranks.
FYI, I'm majoring in Classics and Rhetoric, so I do have some background in this.