Oh in which case if it isn't related to the lunch at all then its legal.
I guess the point is this. If you want to have a big advert suggesting you can pay £X for a 10min flight then that is not legal. However firstly its not the pilot advertising the flights is it - its a 3rd party corporate entity and thus remote from the pilot.
Look at it another way.. what about a wealthy guy in the city with an Augusta 109... he trades equities lets say... and his clients (i.e. people he trades for) give him this business because they like to go shooting/horse racing/watching Formula 1/J Palmer's driving days... They like going shooting with him because he has said 109 and its a jolly good fun...