Enlistment - where?

[Makas]

I was thinking about the XI Hispana or XX Valeria Victrix. The story would either be Boudicca's revolt or Agricola's campaign. But where could this soldier come from? Britannia would not supply it with enough roman citizens would it? I've read about soldiers coming to their legions with their officers after getting money for travel, could it be so in this case? It is after all very far but a legion must need some reinforcements, soldiers get sick and die or have an accident even when there is time of peace.