methinks the plot is being lost......
a wing does not have to be slipping for dihedral to have effect. what do you think corrects for the off centre pilot weight in a two seater aircraft?
the left wing is slightly more level than the right wing, has a longer effective span and generates just a tad more lift and thus makes the aeroplane fly straight(ish)
dihedral also has the effect of changing the angle of attack of each wing in a slip that is self correcting.
ligetti wrote that about 6 degrees of sweep back was equal to about a degree of dihedral. so in the BAE146 the anhedral is to reduce the stability.
not to kill an argument with facts entirely why do tailplanes occasionally have dihedral? :-)