The last SOU-BHD day-trip I did (in mid-Jan) was I would say around 60% loaded on the way up and probably more like 90% on the way back (however we were carrying pax for LCY who's flight had been cancelled due to weather there).
Leaving SOU that morning I was sat in departures next to where the first MAN flight of the day was boarding, and I heard the ground staff give a load of 75 (on a 78 seat Dash).