Warped Factor is indeed correct.... I don't speak for everybody at Heathrow.... However, ask yourself how many go-arounds we have had because the front aircraft was a 146 and because he slowed right down inside 4 DME. Would the problem not be worse with something even slower? How many singles can do 160kts to 4DME?? Once somebody in his 172 corks the speed back inside 4DME the jet behind is going to catch up pretty quickly. Do we need to make the Heathrow Radar peoples lives more difficult with speed control and vectoring, on top of their already heavy workload with weather avoiding and large holding delays?
Alan M is almost correct, except in that the MATS 2 says "Propeller driven aircraft MTOW not above 5700kgs may land on the departure runway PROVIDED IT DOES NOT AFFECT DEPARTURES" . How often during the day do you get a planned gap in departure traffic so that you can land somebody? Everybody moans about doing TEAM, so imagine what it would be like TEAMing with loads of lows and slows.
As mentioned earlier there is the problem of wake vortex inbound (following a heavy requires 7 miles which has to be applied in Class A airspace), and also the problem of departure seps outbound. A standard 2 minute route sep would require 5 minutes for a jet to follow something a bit quicker than a single like a light twin. A few of each of these every hour would soon reduce our runway utilisation and cause some serious delays for the airlines.
From purely an ATC point of view as well there is the added problem of losing the little fellas on the airfield. How many controllers have sat there an wondered where the Learjet has gone to or the Navajo that has come in on a medical flight? By day it is tricky.... by night it is almost impossible to keep tabs on them
I have no bones whether you guys come or not. All get the same excellent service from the radar folk and the tower folk. All I am trying to do is give you a feel for why these type of aircraft are difficult to deal with in that particular environment