It is 508 feet (155.5 metres) to top of the towers, clearance of the bridge deck above high water is 100 feet (30 metres). Span between towers is 4640 feet (1410 metres).
You don't need a plane to kill yourself, around 200 people have jumped or fallen off it, few have survived. It is not practicable to have a suicide barrier because it is an aerodynamic structure. It would be rather like fitting a post and rail fence to the top surface of a wing.
https://en.wikipedia.org/wiki/Humber_Bridge
Engineering Timelines - Humber Bridge
Should more be done to prevent suicides from the Humber Bridge? | Grimsby Telegraph
I would not choose to fly anything other than a hovercraft under it.