Dan for Chrissakes!
So you know for a "FACT" that 120 seconds was not used at Heathrow in the past do you? This is my 25th year at Heathrow and 2 minutes vortex separation has ALWAYS been used. If you read the AIC you posted you will see that the amount of time taken for the departure roll can quite legimately be factored into the two minutes. NOTHING has changed! The separation has been applied that way ever since I arrived at Heathrow.
Of course "it is very easy to get 120s between successive flight paths" and that is EXACTLY what happens! If we waited the full 120 seconds between successive departures we would be wasting 45-50 seconds per aircraft pair. Total that up during one day and that is a HUGE loss of capacity with no increase in safety. Pilots are NOT champing at the bit to depart; they are all too mindful of the dangers of vortex wake encounters exactly as we Controllers are.
You choose to ignore my point about safety regulation - if we were operating in a dangerous manner then SRG would soon intervene. We get VERY few, (in fact hardly any) vortex wake reports in an average week, so obviously we are applying the procedures correctly.
Of course you are touching a raw nerve, because you are questioning the professional integrity of Heathrow Controllers with no hard evidence to back it up!
Differing A340 departure speeds is a training issue. Different companies operate their A340s in different ways, therefore the suggestion that the type is placed in a different speed group would be nugatory.