Disaster Strikes!

The van driver's insurers certainly can write the bike off. It's a simple case of maths. If the cost of repairs plus the current value of the damaged bike exceeds it's market value than they will deal as a 'total loss'

That should be the cost of obtaining an identical bike to the one involved immediately before the accident and that is where the arguments start!
They would probably offer a price which relates to it's pre accident value less the value in the remaining damaged bike.