Bear in mind that every unit has its challenges and the work load at Heathrow could be argued to be just as high as at Gatwick - which holds the title of busiest single runway airport in the world, and the ATCOs there have to work very hard especially in peak times to maintain an expeditious flow of inbounds and outbounds on just one runway - whereas at Heathrow I believe there are several ATCOs working the aerodrome controller position, so less workload individually, as well as Heathrow no doubt being at the forefront of new equipment, making the ATCOs jobs there easier, which may take a while to be introduced to less busy units.
Not quite true.....While there are more ATCOs working at Heathrow than Gatwick at any one time, it doesn't mean that the individual workload is lower, nor does it mean that all the traffic is spread out evenly amongst them. A simple calculation of number of aircraft divided by number of ATCOs doesn't tell one anything. As for equipment, the most radical new piece of equipment (electronic strips) was actually introduced first at Stansted and Gatwick before Heathrow.