Not sure about the UK scene, but I know hackathons for high schoolers in the US can also be challenging. I'm listing a few resources that have helped me below:
* CodeDay - These happen around the US multiple times a year, there's likely one near you. I've helped organize a few of Austin's and they're a great organization. I would reach out to them to see about possibly getting an event in London going, don't know if they're able to expand internationally though. http://codeday.org/
* HS Hackers - This is a Facebook group that was made a couple of years back at PennApps. It's now a community of 3,000+ young hackers. https://www.facebook.com/groups/PennAppsHS/
* HackEDU - Nonprofit started by @zachlatta that's starting hack clubs all around the country. I led ours in Austin and will lead it again next year: https://hackedu.us/
* CodeDay - These happen around the US multiple times a year, there's likely one near you. I've helped organize a few of Austin's and they're a great organization. I would reach out to them to see about possibly getting an event in London going, don't know if they're able to expand internationally though. http://codeday.org/
* HS Hackers - This is a Facebook group that was made a couple of years back at PennApps. It's now a community of 3,000+ young hackers. https://www.facebook.com/groups/PennAppsHS/
* A handful of HS-focussed hackathons: SoHacks (http://sohacks.com/), HSHacks (http://www.hshacks.com/), HackBCA (http://www.hackbca.com/), HackGenY (had international participants this year)(http://hackgeny.com/)
* HackEDU - Nonprofit started by @zachlatta that's starting hack clubs all around the country. I led ours in Austin and will lead it again next year: https://hackedu.us/