Jemy, fair point about the service and expectations, I too would be a bit miffed about this as it does appear to be a bit "money grabbing".
In theory, the regulatory airports would either have included this income in their forecasts when setting the airport charges tariffs, or if they haven't, and it is genuinely "new money", they will be challenged by the regulator in the next review. The CAA have already said that they believe BAA can do better than the forecasts it has submitted for the next regulatory review, ( i.e. non aeronautical income is too low, leading to higher airport charges ).
Bear in mind though that the regulatory regime only applies to the SE Airports, I believe Scotland operate some kind of voluntary regulation ?
Regarding how far each £10.90 goes, it is used to pay for the terminal
infrastructure including forecourts (?), airfield facilities inc runways, taxiways, and lighting, operational staffing ( terminals, operations, security and engineering ) and other activities.
( p.s. I think you mean 4 million £10.90's )