In civil suits, yes. I don't think this is a civil suit. The IPT hears complaints against the government, and I don't think they're considered lawsuits (but may be wrong). I expect that the proceedings are secret though, so's the FISA court here. But this is an American company.
Now, note the second paragraph of the letter where they point out that Apple was told they could talk to the White House, but not Congress. That's a problem. Congress has oversight and investigations powers. They can call the AG into a hearing and won't take, "The UK told Apple to tell me not to say", as an answer. There are things the AG doesn't have to tell Congress, but I don't think this is one of them. Nobody in the Federal government is a party to the argument, it isn't diplomatic exchanges between heads of state, it isn't private executive communication, it's a formal report from a private American company about what another government is trying to make them do. Congress is already allowed to investigate dealings between American companies and foreign governments. They've passed laws about it.
Apple isn't charged with any crimes but would be if they spoke to Congress. If Congress wants to talk to them, and Apple won't, Apple will be charged with a crime. Congress wants to talk to them. Ouch.
