I am not sure iron age British tribes practiced human sacrifice and I am certain not on such a large scale and not by decapitation.
Human sacrifice is one of those things blown totally out of proportion by those who rely on Classical sources. The fact that there is very little archaeological evidence for human sacrifice is usually overlooked.
However I think that such brutality on the scale we see here is very uncharacteristic of the Romans, their most popular punishments being of course slavery and crucification. I await the archaeologists further investigations eagerly!
I wouldn't say it was uncharacteristic at all; maybe the
style of death was unusual but the
Romans frequently brutally punished small groups like this to make an example. Maybe they were a group of high ranking tribesmen, who it would be dangerous to keep alive even as slaves. IIRC it was not common practice to crucify everyone, especially not those of rank as the Roman's
had some respect for class systems.
I think the decapitation has more to do with the Roman's disrespecting the Iron Age Briton's Religious beliefs but I've tried to
type my explanation a few times and it ends up being an essay so I'll leave it for now.