Perhaps they were looking for common attack vectors/device modification surfaces, rather than looking for specific information there.
For example, perhaps the GCHQ have reason to believe that Chinese spys were bugging keyboards and mice sent to journalists and their companies. So they removed the chips that they knew could possibly be bugged and took them home for further inspection?
If they could prove that the Guardians computers were already compromised by Chinese spys, and that the guardian was holding top secret sensitive information on them... GCHQ could skewer the Guardian publicly for releasing state secrets to China.
For example, perhaps the GCHQ have reason to believe that Chinese spys were bugging keyboards and mice sent to journalists and their companies. So they removed the chips that they knew could possibly be bugged and took them home for further inspection?
If they could prove that the Guardians computers were already compromised by Chinese spys, and that the guardian was holding top secret sensitive information on them... GCHQ could skewer the Guardian publicly for releasing state secrets to China.
/speculation