Duty of care
Section 34, Environmental Protection Act 1990
The short answer
The rule underneath all of it. Keep waste safe, hand it only to someone authorised to take it, describe it accurately, and keep the paperwork.
In practice
It applies to everyone who produces, imports, carries, keeps, treats or disposes of waste. It does not end when the lorry leaves: if the person you gave it to fly-tips it, the question asked of you is what you did to check they were authorised.