UK context: a PPL can get free flying, by flying on the business of the company (which does not have to be a limited company) which he is an employee of, provided he is not contractually required to fly and can instead choose to drive, etc.
The company can provide you with the plane, completely free of charge to you. Or you can use your own plane and charge the entire cost of it to the company (Annuals and all).
For the taxman you have to account for business and private mileage separately; that a whole issue but it is separate from the legality of flying on business.
Gcolyer - why would you be stuck in the circuit? I know this is rare (because most people want to do a PPL at the minimum cost) but if a student wanted to fly to say Greece, why can't a PPL/FI come along?
I've wondered about this. I have little time and absolutely no time to do a JAA CPL, but I could certainly fly with PPL students and show them a few useful things. I did an (unofficial and unpaid) radio nav x/c flight just now.