Not a pilot or working in aviation, but presumably the insurance for by a PPL carrier would not cover any commercial activity.
I have read that it is acceptable to accept a contribution for fuel costs when a group share a car journey without the driver being accused of driving 'for hire or reward' or invalidating his insurance. Do it regularly or for strangers and the law would take a different view though.
Don't know if there is a similar relaxation for PPL holders.
During a chat on a train journey many years ago an oil worker told me that he frequently walked into a flying club and asked if he could hitch a lift with anyone going in his direction. It was unsaid but accepted that cash would be exchanged. Said it was faster and cheaper than going by train when travelling from London to Aberdeen.