CBBC has been targeted to 7-12 for a while now. There isn't much content on the FTA children's channels catered to 13-18, leaving a void for linear TV viewing. Might just be me that thinks this way, though.
Realistically programming for that demographic target went out the window years ago. Trouble was probably the closest equivalent as a channel, possibly T4 as a strand as well, but of course neither of these are around.
There used to be the occasional piece of programming aimed at teenagers dotted around various channels but not in any particular strand - Channel 5 used to air The Tribe for example, which was quite successful for them and racked up five series, before it was spun off into The New Tomorrow, and aimed at a younger audience.
I suppose more modern day equivalent is the likes of Sparticle Mystery, I happened to catch the first series of that and I thought it was quite good. The revived Demon Headmaster wasn't too bad either, but that's not supposed to be realistic in any way, shape or form, should have been obvious from the off really in any of the TV adaptations or the books.