When the Romans came here 2,000 years ago they often bought soldiers from OTHER lands (non italian) to fight, then stay as guards etc.
It was too risky for the Romans have say German guards in Germany (in case they all got together for an uprising in their own country).
So many of the guards on Hadrians wall for example were not Italian/Roman but from countries all over Europe (Romania, Germany etc).
Anyway, as has been said rich Romans could well have brought black slaves/workers to the UK and of course some would have died here.
When you watch time team it is amazing how often they find pottery that was from foreign lands, so a huge amount of trade went on between the UK and countires like France, Spain and beyond.