Need help with British (vs. US) English

[dragonyphoenix]

Need some British-speak help. Here in the US there's a phrase, "hand caught in the cookie jar" which basically means you've been caught doing something you shouldn't. I like it for the scene I'm writing because it suggests a kid or someone who is young getting caught. I THINK what we call cookies are called biscuits in the UK. If so, it seems unlikely that you'd use the same phrase. What other phrase could I use?