HeathrowAirport... with respect, methinks you have very little idea of exactly how Heathrow functions. To blandly say YOU think Mixed Mode would sort it out is incredible. You're only a teenager; well, almost 43 years ago I wrote an article in a magazine on "Mixed Mode", or "Parallel Landings" as they called it then, at Heathrow. It wouldn't work then when the movement rate was about a third of what it is now and trying to make it work efficiently now would be a total nightmare. You need to spend a couple of hours on GMC to really understand. The other problem is inbounds, which come in flushes from various directions during the day - sometimes all from the north, then all from the south. In these situations the radar controllers are constantly faced with separation/vortex problems in achieving tight spacing on both runways.
The obvious answer at Heathrow is to scrap the night jet ban and go for H24 operations, like they did 40 years ago. Perhaps you'd like to suggest that to the residents of Hounslow??!!