Amsterdam / Frankfurt / Munich and Vienna offer airport tours which are usually busy. There is just no interest from the authorities at Heathrow/ Gatwick in particular to make this happen or to give the meeters / greeters somewhere to watch the aircraft they are waiting for. The so called grandstand at Heathrow is a joke - open air, low level, no facilities and costs an arm and a leg to park there for a day (cheaper to use hotel car park next door)
With the developments at T5 and now at T2 there were opportunities to offer something that would make a family day trip to Heathrow something to look forward to but they have been sadly missed.