How to say 'I don't know' in many ways

English, and particularly, British English is full of many informal expressions for I don't know.
Here is some of the most common.

• I don't know
• I've (got) no idea
• I haven't (got) a clue
• I haven't (got) the faintest (idea)
• I haven't (got) the foggiest (idea)
• I've got no notion
• You've got me there.
• You've stumped me there.
• Sorry, my mind's gone blank.
• Search me.