Hi LC, glad you like the service.
On Heathrow North (119.72) there is very often a second controller sharing the frequency. This support controller's tasks are to descend aircraft in the stacks, telephone co-ordination, answering first calls (checking types, cleared levels and ATIS received) and deciding the landing order. This allows the radar controller to concentrate on vectoring off the holds at the right time and to the correct position.
If holding for more than 5-10 minutes there will usually be a support controller on 119.72 but we rarely employ a support controller on the south frequency (134.97). If you've heard two voices on 134.97 or 120.4 that's almost certainly a trainee and mentor.
Gatwick and Stansted use the same system. I'm sure it can't be just a UK procedure but having demonstrated it to Italian and French controllers they couldn't understand how we knew when to talk and when to let the other guy talk.