for a -500
Okay Gatwick, let's assume runway 26L (don't fly out of LGW, so no idea if this is the favoured runway for departures), no wind, standard atmospheric pressure and temp 20 deg C. Weight is 53000 kg, which I guess would be about right for a full-house to MAD
Vr (speed which you pull back on the control column) = 136 kts or 156.4 mph using a 1.15 conversion factor.
Hope this helps.