The existing 3rd runway is all but dug up these days. Some bits of it are taxy ways and others, stands.
A-CDM has now come in at LHR. It already exists at several European airfields such as CDG, FRA etc. To ease congestion at holding points, aircraft are issued a TSAT or target start up time, based on there EOBT (estimated off blocks time). With a local 'fudge' factor for airport specific bottle necks, the crew call up for start at the allocated TSAT and get underway. Sometimes TSAT is STD, other times it's 12 mins after etc. It depends on the local fudge factor and any other enroute capacity delays (slots).
Many carriers taxi in on single engine after a cool down period of apron 2 mins. Some carriers even taxi out on one engine. Depends on type and operator. At 120 USD per barrel, airlines are doing every thing to save fuel.
As an earlier poster asked, why not hold aircraft on the ground rather than in the air, far beter co-ordination is being seen in this area, expect more as time goes on. Many flight get speed or level capped to sequence arriving aircraft early on. Sometimes very early on into a flight. FRA is a good example.