It could be Flybe to get volunteers.

But to sell ticket at £0 return was for years. Aer Lingus advertised on Metro and Evening Standard for at least two years sometime between 2009-2012 (can not remember, but I do recall it was a long campaign). Of course you have to pay tax and charges.

I seems to remember such practise (volunteers to sit in plane) was mentioned in Evening Standard commentary piece some years ago (possible 2010, 2011?). Can not find it online now. But I am 85% sure the author said it was Aer Lingus did that in order to keep Heathrow slot thus not to cause ghost flight situation (as BMed and BMI was heavily criticised in 2007/2008 for ghost flight). If not I apologise. But before I can find that article again, please allow me to insist that my memory was correct.