<<I understood that LHR ATC have a standard pro forma that they read out to aircraft in distress notifying/asking them of the risks of overflying built-up areas?>>
Would someone from ATC confirm this please? The only arrangement I am aware of is that in the case of a major problem involving an aircraft, e.g. something which may prevent it from landing and leaving the runway, ATC maybe asked to relay a message from the airport authority asking the captain to consider diverting to a less busy airfield. Of course, the captain has the final decision.