London don't need to for city departures except 09 BPK and CPT dep. London probably do it out of habit as most departures in the London TMA are transferred to them from the towers after departure and not to the approach sector exceptions being
KK: Wizard, Tiger and 08 SFD
LL: 09 CPT
SS: All westerly deps
GW: 08 Olney and CPT
LC: All work thames except 09 BPK and CPT