Where can a 12-year-old boy find a job-in-kings-Lynn?

Essentially nowhere. Child labor laws severely restrict the ability of companies to hire anyone under 16, and fairly severely restrict the employment of those under 18.

Given that, and the current high unemployment rate, only highly specialized businesses (for example: the entertainment industry, if they need an actor to portray a child, and even then they typically strongly prefer to hire slightly older actors who LOOK young) are going to put up with the restrictions and regulations involved in hiring a 12 year old.

Your best option is to go into business for yourself, either with a paper route (though that's not as viable as it used to be back when most people actually subscribed to the newspaper) or doing odd jobs like lawnmowing or shoveling snow.