Remember : we don't know what happened, we know almost only what Caesar said, and often we can see that he said only what was good for him. For example, when he says that he captured an oppidum swiftly, he don't say how big it was. If he captured a big oppidum, he don't say how much time it took to do it. He says here that the Helvetii wanted to move from their territory because they were unable to make war as easy as they wanted to do it, but he says there that these Gauls are continuously fighting against Germans.
About Britain, Caesar could have said "I came, I saw, I left". The operation was destined to impress the Romans (This operation was indeed as amazing as a landing on the Moon), but it had no concrete results.
After all, I'm not sure that Caesar was "Against the Celts" : he was first of all "For Himself". And his cavalry, even during the Gallic War, was almost constantly made up of Gauls, except at the very end, when he enlisted... German horsemen!
Note also that "the Celts" didn't form a coherent political or military organisation. Each Gallic state (civitas) felt free to act according to its own interests, even against other "Celts", except during the Vercingetorix episode.
I think that Caesar was a filou