Can you explain your situation a little more?
You cannot be convicted of having drugs you if you didn't. The Crown has to prove that you were actually in possession and a bag of something on the floor of toilets in constant use by people in a busy nightclub would appear to be fairly flimsy evidence. You do of course take a risk by refusing the fixed penalty and being prosecuted but from what you say there would appear to be insufficient evidence against you. Happy to discuss further. I hope that helps. Please leave a positive rating so that I am credited for my time.