I did this. Sort of. I was 15 and my family died, so I legged it instead of becoming entangled in social services.
Don't do it. It is an incredibly hard thing to do and not suffer a lot while you're doing it and beyond. It can have repercussions for many years after. It's opening yourself up to a word you're not used to, or capable of dealing with in reality. At 15 I knew nothing, despite my belief I had it all down. I waded into very very deep water and I didn't even see it happen until I was in over my head.
Life is expensive. At 15 you can't afford it. Or get a job that can carry you. Where is rent coming from? Where is a daily meal coming from? Clothing? Transport? Taxes and utilities? None of this is happening, let alone finding a landlord that will take a 15, 16 or 19yr old without a lot of assurances you can't give. So now you're homeless with no work.
Later in life when you are trying to find employment and it becomes clear you left school at 15 with obviously nothing, nobody will give a shit why it happened. Next candidate pls. If you're lucky enough to get off the streets largely unaffected.
It's incredibly lonely, being essentially destitute. Your mental health will suffer due to this and so much more. The way you are treated, the things you're needing to do to get by that you never would agree with before. Why do you think homeless people drink or use drugs? It's that or suicide for many.
You'll meet friendly people, but they're in a similar boat to you, lower end of society fighting for a step up and they have angles you cant see or wouldn't suspect. Friendly isn't always safe, believe me. Being homeless and jobless, essentially starving changes everything and everyone it touches. You will turn to whatever means to get by.
You know what, a reddit comment doesn't do it, I can't fit it in here just how much you're harming your self by opting to slip through the cracks. Life is hard, don't opt to make it harder in ways so cruel you can't yet fathom. It's so complicated and tangled it could be an entire book. If you have any choices at all, and you have in the UK, take them over this.