Seems like they were following US DoD destruction guidelines for classified information systems. GCHQ probably uses the same directives, probably all of the five eyes do in all actuality.
Anyway, once classified information is introduced to an information system, any peripheral attached to it at any time after the introduction of the classified material becomes a classified information system as well (this includes keyboards, mice, monitors, printers, scanners, webcams, packet radios including Wi-Fi and bluetooth adapters, wired NICs, modems, routers, chargers, you name it) and is subject to destruction.
Honestly, I'm very surprised they let the Guardian see the results, seems like a security risk to me. I would have had the equipment incinerated had I been running that operation but thats just me.