In a situation like they were in then disruption is not even considered. The pilots would need to decide what to do, where to divert etc. As it was a significant problem then disruption to LHR's passengers is irrelevant. Priority is get it on the ground safely. ATC don't tell the pilots where to divert, the pilots make the decision and they tell ATC where they are diverting. ATC can offer help (and in the UK are very good at doing so).
As for not flying over a populated city, losing both engines could happen at any time anywhere, it's just very unlikely. BA38 incident at LHR for example. The pilots would have been busy enough dealing with the ECAM and handling the aircraft, their priority would be to get it on the ground safely and keeping their passengers safe, as they weren't in a glider scenario then they wouldn't have been looking for a green field to land on. Perhaps it was considered, even momentarily, but given one engine seemed to provide thrust (fire included or not) meant they could land on a runway.