They crossed a line when they did this to a military base...
They didn't cross the line into terrorism, though. Yes, they are criminals, no, they are not terrorists. It's an incredibly important distinction. By definition, they are not terrorists:
criminal acts, including against civilians, committed with intent to cause death or serious bodily injury, or taking of hostages...
-definition of terrorism, UN resolution 1566
Millions of pounds? Oh those poor pounds, they must be terrified! Wait, what? Money doesn't have feelings and nor do fighter jets? Therefore no terror was caused, and the proscribing of this group as terrorists is therefore absurd? No, no. tHeSe DeFeNsE cOrPoRaTiOnS hAvE a RiGhT tO dEfEnD hErSeLf.