Heathrow Perspective
As has been stated, the B757 in the UK is classed as an upper medium, therefore departure separation for a lower medium or small behind a B757 is one minute, (i.e. "wheels up") on diverging routes. Pilots are entitled to ask for extra separation, but please inform the Controller BEFORE you line up - s/he may change the order to improve expedition, or s/he may be relying on you rolling promptly with landing traffic on final.