Thats a bit complicated! Out of Heathrow ATC will often clear you to a higher altitude sooner if they know you can be above the holding stacks before you reach them, otherwise you get a long track at 6000 ft until you've passed beneath them. I know certain airfields in Germany have long SIDs in order to ensure you reach a certain altitude before the FIR boundary. The sooner you get the altitude the sooner they can clear you on a much shorter direct routing off the SID.