Absolutely. What we would now call southern England and northern France and the Low Countries were extremely well integrated, to the point where the Romans identified a significant chunk of the island with the Belgae (a northern Gallic community) and archaeologists until the last couple decades believed there had been a "Belgic" invasion of southern England in the late centuries BCE. Julius Caesar himself justified his landings on Britain by claiming his Gallic enemies were receiving help from the island, and while some have considered this a bare pretext, I personally think the rather large army that met him at his landing site give some support to his belief.

Now whether there would have been any British slaves specifically in Rome around 70 BCE is a bit harder to answer. There was a very important trade link that ran from modern Marseilles up through Gaul to Britain so it is possible, but probably would have been very rare.