Well, generally speaking the celts didn't exactly use much tactics in their wars. Their main tactic was a mass attack hoping to overrun the enemy, but they were also famed for their ambushes.
However, under certain leaders of more quality they did much better. Ambiorix used cunning to draw the enemy out of his camp and ambushed them, killing more than 1 legion.
Vergingetorix knew he couldn't beat the romans in a head-on battle, so he ordered the cities, villages and crops to be burned. In the end Caesar and his legions proved to good for vercingetorix, but he did make it quite hard for Caesar.
And indeed Cassivellaunus also used more advanced tactics and strategies, but again: Caesar was carried the day.
It seems likely that the chariots were used for skirmishing, but according to the commentarii their main tactic was to dismount, fight on foot and when pressed their chariots came back, so they could quickly withdraw.
The british invasions were a rare occasion of Caesar being badly prepared. Especially crossing the channel and finding a proper landing place proved a difficult task.
Caesar almost certainly exaggerated in his commentarii, it was meant to make him popular in rome. But the broad picture is accurate. It would make Caesar look very bad if he really was lying, because many of his officers, including Cicero's brother, wrote letters to people in rome. His lies would soon have been revealed. So his commentarii try to give an account favorable to Caesar, but it could not depart from the truth too much.