Regulation 2 - Interpretation,
7. For the purposes of these regulations:
(....snip)
(d) an aircraft that is flying or operating for the purpose of, or in the course of:
(....snip)
(v) the carriage of persons or the carriage of goods without a charge for the carriage being made other than the carriage, for the purposes of trade, of goods being the property of the pilot, the owner or the hirer of the aircraft;
(snip...)
shall be taken to be employed in private operations.
If you can get esteemed regulator to define exactly what 'for the purposes of trade' actually means in this clause you will have your answer of if you can do it on a PPL. Good luck!