EI-BUD
BHX is currently at 8.7m pax, it was as high as 9.6m a few years ago.
The current facilities could handle twice this amount, so yes well under utilised.
We should be in the 17m-20m pax range by now on par with STN & MAN.
As you say Birmingham is the second UK city and its airport has historically under acheived, hopefully things will pick up soon and with the forthcoming runway extension I do thing long haul opertors will see BHX as a London alternative.