Thank you!
It's really hard to find a job here :( (I also live in the UK) and any job I applied to, told me to call them back when I reached 18.
So I don't really have an answer. Actually, you might be lucky. Some places do hire under eighteen year olds but I was hired a few months before my 18th birthday. You could try and get a job at a local shop etc.
Good luck.
News-paperboy? oh wait, it's 2015; never mind. Use the internet. Make a website to design stuff for other people or... Babysit! Yeah, even if you're a guy just babysit – it'll pay the bills (since you're 15 you have none anyway) so yeah. that's what you should do. Or become an underage prostit---- i mean pro stitcher at a hospital.
You are 15 you have a whole life ahead of you. What I recommend is to start an online business. You will not make money maybe until 6 months later, but once you start making money you wont stop. You will be making money from anywhere as long as you have internet access. Check and see here inside WA, don't worry, its free https://youtu.be/oBQAY3tf8pY
Start your own business. Craft and sell it online, somewhere like Etsy. See if anyone needs yard or house work done. Tutor. Trust me, there are ways, if you are willing and able.
