As mentioned by ATCO Two earlier on in this thread, the A340 doesn't need to be in a slower speed group. The speed differential between them comes solely from the way the airlines operate the aircraft and to what extent they derate the engines for departure. Any controller with any appreciation of aircraft performance will monitor the ground speed and rate of climb using the ATM so that we don't put something faster up his chuff. Only the other day I launched the Air Mauritius A340 on a Dover departure and had to wait almost 4 minutes before I could go with another southbound due to the poor aircraft performance and the strong headwind that he was encountering. The same principal applies to the BAe146... Compared to most jet aircraft it is slow on departure, and so we don't blatt another aircraft off as soon as its wheels leave the runway!
As for the wake vortex issue, I think you will find that all controllers give a full 120 seconds behind a heavy departure. Feel free to monitor the seconds that pass between the nosewheel lifting of the first aircraft and the nosewheel lifting of your aircraft.... Always 120 seconds!! If you want more, then ask!!! But please bear in mind that if we are operating single runway procedures or we are taking landing traffic on the departure runway then there may be something screaming in down the approach!