If this doesn't work you might want to get in touch with the
AUC, who have been known to intervene.
Just to add to this point, the AUC no longer deal with aiding passenger complaints towards airlines. Pity, as when I had call to use them they were very helpful. However, the CAA is now taking over this role,
CAA strengthens passenger representation and streamlines complaints system | CAA Newsroom | CAA
There is an email for the CAA on the link provided by ExXB.
As far as suitcases go, I use Samsonite hard shells. Very strong and sturdy, never had a problem with them being damaged.