In general, if a/c are follwing the same departure route, or a different route that has less than 45 degrees difference, then the separation is two mintues. If the succeeding departure routes diverge by more than 45 degrees, we can use one minute, or 'wheels up' separation.
Next time you're at LHR, have a look. If one a/c rolls immediately behind another, see which way they turn at two miles out.