I think it really depends on you as a person, as well as which subjects you're doing, and what kind of HE institution you go to. If you go to a school sixth form, it's a lot more like school. In the school I used to go to, the sixth formers have to be in from 8:45-3:00 every day even if they only have one lesson, and if they're not in a lesson they have to be in the silent, supervised study area.
I go to a sixth form college, which is a mix of academic and vocational subjects. In general it's very chilled out here, a bunch of people skip lessons regularly (though I know people who have been kicked out due to this). Academic achievement is, in general, a lot lower, as is pressure to succeed, so at times it can feel as if there's an atmosphere of failure where it's seen as completely okay to fail. On the other hand, it's been a really clean break from my previous school, which I hated, and people are, in general, more accepting. We call our teachers by their first names and they're very encouraging towards people who really want to succeed.
In terms of subject difficulty: it is much harder than GCSE, but if you're a very talented person in a couple of subjects then you'll find it easier than other things. For example, I got 8 A*s and 2 As at GCSE, so I'm taking four subjects at the moment. The subjects I have been able to actually succeed in are either AS subjects (Ancient History and Classics, considerably easier than linear subjects) or History, which I like to think I'm very good at. English Literature is something I have really struggled with, partially due to poor teaching but also because I've struggled so much I've found it hard to motivate myself.
My biggest tip for managing the step up in difficulty is doing lots of work. It's very very very tempting to use all your free time to just hang around with friends, but if you actually work your hardest you'll find A-Levels a lot easier to manage.