This question is partly inspired by watching the TV documentary The Celts: Blood, Iron and Sacrifice in which Alice Roberts and Neil Oliver show how the Celtic leader Brennus sacked Rome in 387 BC, followed by the defeat of the Celtic leader Vercingetorix by Julius Caesar at the Battle of Alesia and the last stand (as they see it) of the Celts in the rebellion of Boudica.
Roberts and Oliver say (in the second episode of their series) that, when Caesar invaded Gaul, the tribes there were initially divided among themselves. At least one tribe had traded with the Romans for almost a century, so some tribes were relatively friendly to the Romans while others resisted.
As Caesar's campaign proceeded, the Gauls united behind a young, skilled general, Vercingetorix. He initially used guerrilla tactics with some success before being trapped in a hill-fort at Alesia, leading to his defeat by Caesar at the Battle of Alesia. Roman engineering - the construction of a two walls, one facing inwards towards the hill-fort and the other facing outwards for defence against the Celtic relief army - seems to have tilted the battle in favour of the Romans. Perhaps the better logistics, organisation, training and engineering of the Roman Army would have defeated a united Celtic army in any event. In another historical discussion thread, discussing how the Gauls were effective against the Greeks, JaM said:-
This suggests that the Celts were not as disorganised and undisciplined as we might have thought. Perhaps Vercingetorix made a strategic error in allowing himself to be trapped in the hill fort at Alesia. If he had continued to use guerrilla tactics, could he have worn out the will of Caesar to hold Gaul - or was the domination of Celtic lands by Rome inevitable? At the Battle of Bibracte, for example, six Roman legions fought an (apparently) much larger force of Celts - despite being outnumbered, the Romans inflicted a devastating defeat on the Helvetii, even though the Boii and the Tuligni marched to help the Helvetii. Such victories seem to suggest that Celtic tribes did not stand a chance against the Romans.
But perhaps the Celts could have used different tactics - or, if the Celts had fought the Romans earlier, maybe that would have made a difference? Was there a point in the development of the Roman Army (such as the Marian reforms of 107 BC) after which Celtic armies would have little chance against Roman ones - except for unusually favorable conditions such as the ambush at Teutoburg Forest in 9 AD (the battle which famously caused the Emperor Augustus to cry 'Varus, give me back my legions')?
For anyone who has interested, I hope to explore this question in an Iceni AAR, Andraste's Children. Perhaps, if the Iceni can unite the tribes of Britain and then some other Celtic tribes under one banner, they can resist Rome. If anyone has ideas on whether, historically, the Celts could have resisted Rome, then - if I use them in the AAR, I will credit you.