It is all worst case senario, and also for RTF failure procedures to ensure the departing aircraft are well clear of the stacks/inbound flows before commencing climb as per the failure procedure. As mentioned above, LAMP will begin to be introduced (3 or 4 stage implementation) to improve this. At present, all aircraft have to fuel up for the worst case senario, a heavy going longhaul on a DVR SID burns ridiculous amounts of fuel during the departure - carrying extra fuel just to be able to carry the worst case senario fuel. I've seen the figures but cannot remember exactly so not going to guess, but it will be millions saved in fuel cost PER ROUTE each year (based on a full B747 to Singapore departing on the DVR SID every day off 27L/R). Stage 1a of LAMP introduces point merge approaches to Gatwick and London City & will improve departure routes from London City, Southend, Biggin Hill, Standsted & Luton. The knock on effect is more possibility for earlier climb departing Heathrow & Gatwick. I believe the SIDs off LL & KK will be shortened to end at DET too so the fuel carry will be slightly improved. It's a long & gradual process but eventually (2019??) all the changes will have been made & the much better profiles introduced.