As said hypothetical pilot was in receipt of a FIS from Farnborough, the FIS controller (may all his lottery tickets be winners) asked our "hero" if he realised he was infringing Gatwick's zone and politely suggested that he might like to make his heading 270 degrees, before the situation became any worse, preferably fairly promptly.
Or the hypothetical pilot in receipt of RIS from Luton who, having abandoned his plans for the day and trying to find his way back home through significantly worse than forecast weather, decided to go over rather than under the next little cloud and found himself a tad too high ... the controller was quite nice about it when I phoned him later.