Search the forums. The question comes up every now and then.
The two most researched options are parachute dropping and glider towing. Within a club environment, those are the two things that the CAA allows others to pay for your flying, instead of having to share, at minimum, an equal part of the cost like normal PPL flying.
But experience has shown that parachute and glider clubs do not want new PPLs to do these kind of jobs. They are looking for people who
- Have experience with the activity involved. Eg. 100 parachute drops yourself, or a Diamond C in gliding (whatever that is)
- Have at least a few 100 hours experience as PIC
- Are willing to commit a serious amount of time to the sport, since the conversion/checkout to flying a high-powered parachute drop plane or a glider tug is a serious investment.
Doesn't prevent you from trying to find a place to do that though. But if your only motivation is "I want to build hours for free", I doubt whether a lot of clubs will take you on board. There's no shortage on people who want to build hours for free...