A pidgin is essentially a failed attempt at multilingualism? For some reason, the natives get away without being made to learn the dominant language properly.
And having contracted some commercial transaction in the necessarily-simplified pidgin, what do you do when you go home and want to talk about something deep? Use your mother tongue?
[NB. The suggestion in Underworld is that the Indus Valley script was used only for commerce and such prosaic uses. As and when it gets translated, it will belie the complexity and subtley of the langauge actually spoken.]
In Scandinavia or Iberia, the languages are already sufficiently close to be mutually understood without a pidgin intercessor. Likewise, surely, no pidgin would be required in Britain when the languages and the people were not far removed from each other.