Misuse of Drugs Act 1975, s 7(3)
It’s a defence to a charge of possessing an illegal (“controlled”) drug if you’d taken it off someone to prevent them possessing, using or dealing in the drug. As soon as possible you must have taken all reasonable steps either to destroy the drug or to take it to the police or someone else who’s authorised to have it, like a doctor.
If you’d simply found the drug, it’s a defence if you’d taken it with the intention of delivering it to the police or another authorised person, and if you took all reasonable steps to do this as soon as possible after finding it.