Vfr,
Orthodox UK mil teaching discounts the windspeed, as Nick Lappos mentioned - all downwind departures are treated the same. A hover check downwind is carried out and the power noted, then the max power available is confirmed. If the difference between the 2 is 20% or greater, then the power margin is considered adequate for a downwind transition. However, all the comments appended by NL are agreed - we only carry out downwind transitions if there are over-riding (that's not flying across Yorks, by the way...) operational factors or considerations.