Darryl managed to understand the individual words, but not the meaning of the sentence as a whole. In fact, this means that we REALLY stole a lot of money. 'Not half' is a British slang intensifier. The speaker is delighted that they managed to steal so much money.

This sentence requires some picking apart to start. "Nick" or "nicking" is the action of stealing (the word is probably British in origin). "Quid" is another description for "money". So, the straightforward meaning of this sentence is:" We haven't even managed to do half the action of stealing a few British pounds". Or to put it another way, "we haven't even managed to steal any money". (Edit: Suki is right- it looks as if "not half" means "a lot of" in this case. I was wrong in my understanding. I'll leave my comments here as a reference for the other readers.)