Apart from the operating crew and those in receipt of the ASR no-one knows what caused the smoke and fumes. (Probably only the engineers know at the moment! :-) )
In most Boeings some of the cockpit air comes from the avionics cooling bay and thus will pick up anything that might overheat. The smoke and fumes checklist is designed to isolate various systems not critical to the immediate task of flying the aeroplane in order to stop/reduce the emission of fumes/smoke.
If the checklist is successful then you will have reduced the smoke problem and be left with an aircraft with potentially degraded secondary systems but one that is perfectly flyable.
Hence there is no reason why, in those circumstances, you should not return to your main operating base for the convenience of your customers. Even if that means going 'straight over the top of central London' in a serviceable aircraft on a standard approach to Heathrow.
