Woh there Dan,
Vortex separation is 120 seconds!!! Regardless!
At the two busy SE airports I've worked at, vortex is taken very seriously.
Professionals tend to be picky like that!
The two mile launch you refer too is to separate aircraft on the same routing....for aircraft of the same vortex or higher, and of the same speed. That rule of thumb provides lateral separation at the end of the SID, it is not sufficient to ensure vortex spacing!
Grasscutters rule on rotation to rotation is correct and has been the subject of much discussion on this BB (e.g when does the wing start to generate lift and therefore generate vortex?) and it is a technique taught at the College of ATC.
As for sitting on the threshold longer at STN than LHR, do STN have to request a release from radar before they can launch the departure, where LHR & LGW have free flow? (I'm sure someone from STN or Essex will reply sometime)