Thanks Doug and Niku. There's no way I can take my harp on board. Even my smaller harp is just about too big for the overhead bins. What I did with my small harp last time was loosen the strings a half step and packed it so tight with clothes both inside the soundboard and all around the harp until I could barely get the case zipped, and then I checked with the hold luggage. It was a huge risk, but it came out the other end at the Manchester airport absolutely fine.
I won't be able to get a flight case made for my big harp like MynameisCat suggested, because the cost is phenomenal.