LHR with 125mppa will need 4 rwys, no getting away from it
I don't know where you dredge your figures up from, but mine come from BAA !
As I have pointed out before, BAA told the 2011 GLA Plane Speaking Inquiry they already have planning permission to increase LHR pax numbers to 95mppa; this is from the T5 / Heathrow East proposal (T2A, T2B & T2C).
Tackling air and noise pollution around Heathrow | Greater London Authority
However BAA have revealed further plans for T2D, T5D & T5E (not to mention Toast Racking the eastern end for T3/T4); if you accept BAA's proposals 125+mppa looks achievable.
Using BAA figures the present T5A, T5B & T5C have a maximum capacity of 35mppa. In addition to which we now know BAA / IAG want an enlarged T5A + T5D + T5E and are making provision for a T2D; working on the basis each extra satellite will add 10mppa this will add another 30mppa to the existing planned 95mppa giving a total of 125mppa.
Then adding on an eastern airfield redevelopment it comes to 125+mppa; I'm only using this low figure due to the slow increase in passengers / plane; to speed this up I am proposing the decantation of small planes to NHT which will additionally free up main runway slots for new destinations !