They split gnd into 121.7 and 121.9 because EGLL is so busy. You'll see the demarcation line on the gnd plate (I don't have it infront of me but I think it's on plate 10-9) between Gnd North and Gnd South. Possibly try asking the same question in the ATC forum if this didn't answer it, there are some EGLL ATCOs there that will definitely help.