you can text 'shout' to 85258 for free support
but i rlly rlly would recommend speaking to a parent or trusted adult in the long run,, i'm not sure what your reasons are for not telling your parents but if it's safe to do so i promise it will help !! you don't even have to go into detail with them just make it clear that you need professional support and hopefully they are able to help you get that
in the meantime you could also speak to friends -- letting someone in is the main thing and the first step to getting better