Internet ressource for english-british expressions

Hello,
I'm looking for something rather specific and I hope that someone here on the forum could help me. I'm willing to improve my current English (so as to pass a Cambridge exam one of these days) and it occurs to me that I suffer a lack of knowledge for specific expressions (idiomatic expressions ,). The tricky point is to explain clearly what I'm looking for !!
Let's give some examples :
"to fall head over feet"
"It occurs to me"
These kinds of constructions where the meaning can't be (easily) deduce from the meaning of either the central verb either the central substantive ! I hope I've made my point a little clearer ?

Do you known an Internet website (better if free of charge) or a dictionary I could be able to find in France (yes, you should have point out my gallicisms already : I'm a true Frenchman) with specific attention for such, both colloquial and literary, expressions ?

Thank you for your attention,
Regards,
Yojik