I think there is a bit of confusion about rules here.
a few years ago, Maldon had some 40mm targets that were half obscured. Ian had plates welded to the back of the targets so that you could only use half the kill. The problem was, once the paint had been shot off the metal plate, it looked like a full size kill.
The UKAHFT put a statement in the course setting notes, "Sent to all hosting clubs" that if a plate was to be used to obstruct a kill, it had to be on the front of the face plate, so if the paint is shot off, you can still see the obstruction.
The targets used at Cosford were not 50% reduced kills. They were the targets that can be either a 25mm, 20mm or a 15mm etc
As far as I am aware, these do not fall into the same category as the half obscured targets and as such are fully legal