I'm going to say the same as all of them. Do not instruct for free. It's not fair to other instructors and it undervalues your skill. One other option is to take payment in flying rather than money to keep up your P1 time.
All schools/clubs need instructors. They all seem to welcome part-timers especially at the weekend to cover the glut plus the fulltimers like to get the occasional weekend off. No school minds paying the going rate £15-20/ hour. If they do go somewhere else.
Clubs churn instructors all of the time- mainly due to the poor pay, you should have no trouble locating a position.