Not sure how you do it in UK but in US it is a little loose too. If there is a Geological Survey datum on the airport that Lat/Long should be it. Where there isn't I understand the center of the airport or the base of the control tower is the place. It should be the same place the airfield elevation is determined, in any case.
In US the data for the airport is provided to the map makers by the airport owner/operator/sponsor. Make for some strange data coming in (a lat/Long plotted that is in another state. Periodic update of the data (a check every few years) results in some airports moving. (hmmmm)