I've never worked at Heathrow, but as far as I'm aware, Heathrow Ops have the final say on runway in use, and ATC have to ask their permission if they wish otherwise.
I was once told they use 09R for landings only rarely because of traffic from Cargo and T4 using the 'southern' taxiway affecting the ILS glidepath signal as they have to taxy between the aerial and the landing aircraft.