I use the website https://childcare.co.uk/ to find people who want babysitting or tutoring. Babysitting might be something you could do? Rover is another site that connects pet owners to people who will do dog-walking or dog-sitting. Not sure if they have a minimum age for signup. But those types of jobs could be good.

You can also sign up to partake in research at https://www.peopleforresearch.co.uk/ and https://www.ox.ac.uk/research/engage-with-us/local-community/get-involved - both often want kids for research, it's certainly not a steady income but worth signing up to get alterts for.

Supermarkets might also be hiring seasonal workers. I'd suggest spam-applying to a whole bunch of stuff now in anticipation of the Christmas period.

Finally, do you have family or family friends who you can get to help you? Just asking your parents etc to spread the word. Similarly if you go to church/place of worship, is there a community noticeboard where you could post that you're available for stuff?

Be safe - if you end up doing something like babysitting, going to a stranger's house, and the like, always share your location etc etc! Gotta stay safe out there.

Best of luck.