I count myself lucky that I have a good command of the English language - my writing skills have been my bread and butter for years. That said, I had an unconventional schooling experience which was extremely literature orientated.
Part of writing well is having broad exposure to an array of prose, not just literature which I don't think high school students get enough of - and if they do, they hate it, so it's not exactly helping. It's not enough to put sentences together, you need to have vocabulary also. But if I was forced to read To Kill A Mockingbird (for the sake of example), instead of getting to drop it and choose a different, more interesting book, I would have learned nothing and been miserable. I had a strong voice entering university, but it's taken the four years to develop - journalism and everything else were great for that, writing every day on top of school assignments.
All that said, my first year at uni I had to take an English course (I AP'ed out of two of the three required courses) and it was absolute hell. People did not know how to write. What you're taught to pass tests and pass high school English is this absolutely horrific five paragraph formulaic essay wherein each five sentence paragraph has an introduction and conclusion. You're essentially restating yourself for all of the essay. Total garbage.
I don't know if that same formula is taught in UK schools, but for the love of all that's good and holy I would hope bad writing is just because students aren't writing - not that they're being taught something which won't help them in the real world.