Kag, an interesting post and a quick search reveals this:
BBC News - Graduates paying to work for free
What is interesting is the comment at the end:
"The government's own lawyers have warned work without pay is often illegal and HMRC should be investigating companies which offer unpaid and paid-for internships."
Given that easyJet are the main culprit of this in recent years and the numbers that went through on the P2F scheme there could be quite a large sum in question involved. IIRC there were in the region of 40+ P2F pilots paying in the region of £10k (that was the figure being suggested) so no paltry amount in total; £400,000
Should HMRC decide to investigate I imagine some senior heads could roll.