Erm, if you must know...
It's secured to the vehicle with an overload break DROPS drophook and designed to do exactly that.
Also, the empty vehicle would fit (It's on the other side), it is possible to forget you have a load on the vehicle, we are all human ya know...
He will well and truely have the book thrown at him and will probably be returning to see the instructors at Leconfeild and be the laughing stock of the re-training course, and will likely receive points on his licence like any civil driver.
And them things aint fast, so yeah...
Anyway enough defence of the silly **** now, he should have:
1: Known the height of his vehicle with load, the cabs of these vehicles have adjustable reminders for this very purpose.
2: Better secured his load, using the hook only in Basara when you're loading an MLRS is all well and good, but in this country, you have time to strap the load into place on your vehicle, and place the drophook into 'locked' mode.
3: Known the route, relying on Sat Nav for military traffic is ridiculous...
PS: I've been in the RLC (Really Large Corps) and trained at Leconfeild, I know that these things are covered in MoD vehicle operator training!