it is very sad, best wishes go out to the friends and family involved.
18 are not particularly valuable... they can be replaced. most have been re-chassises a few times, and will be a few more times. i doubt the car will go away with this unfortunate and sad accident.
i also don't agree that the cars were likely run too hard. they are race cars, and they get raced.
the one things though - some (i don't know about this one) euro and specifically english events - are run without modern in car safety. there is no good reason to not prep / modify these cars to pass a broomstick and run with a hans, to have a fire system and elec. cut off
accident happen... IMO its up to the individual to decide, and should be able to chose if they like to run without belts even (recent "revival race where a driver got thrown out in a crash!?!) but, fi your gonna run with lower in car safety... be prepared...
i don't know the details of what happened here though.