To expand on this, I have seen material from the CAA that essentially says that it's OK for a PPL to take colleagues, customers and whatnot on a private flight, IF the flight is only incidental to the trip.
I'm not disputing that as an interpretation, but it's worth being clear about what the law says the condition is:
"Neither the pilot in command nor any other person who is carried is legally obliged, whether under a contract or otherwise, to be carried on the flight."
That's all.