You need to check the labor laws in London to see at what age you can work legally. I know in the US you cannot work officially until you are at least 14 years of age. You could work at your family's business. Like if your parents or relatives owned a store, restaurant, farm, or something like that.
You could try volunteering somewhere at this point so then you can have some work experience for your future job applications and resumes. You could offer your help at places like a church, community center, day care/child care, or maybe even your school.
Most 14 year olds get their first job at places like fast food restaurants, amusement parks, libraries, or government sponsored programs. Hopefully your birthday is coming up before next summer starts. Then you should be able to get a paying job by that time.