This. I add to it sometimes, "I don't want to tell you something now then go back on it [when I thought about it properly]".

Because how do you challenge this? Insist someone should know an answer without thinking? Insist their guess should be the same as a thoroughly thought out answer. At the same time you're not being evasive, or saying that you have no idea. You're stating you want to deliver a high quality answer first time round.

Even if people push back on this, you made it clear you're offering a guess, not a considered opinion. Nothing wrong with providing a clearly labelled guess, then changing it later.