As a sixth former, I have no take on colleges but I can share a lot regarding Sixth Form -- granted the specifics will likely differ on a school by school basis, of course.
Sixth Form is a lot like school but with more privileges/lax rules compared to the rest of the school. It's split into two years, Year 12 and Year 13. If you take A-levels, this will typically be 3 or 4. Typically the 4th is Further Mathematics, some schools require you to start with 4 and drop one later on. I personally am an advocate for starting with 4, granted it wasn't compulsory for me.
While you're allowed to drop a subject at more or less any time in Year 12 (Year 13 is a lot more iffy because the subject may be down on your UCAS application and universities may have given offers regarding that subject or the like), you can also take up another subject if you feel like it. I personally took up an extra subject after a half-term, and I recommend doing it before holidays if you choose to take an additional one, as you can spend that holiday catching up so you don't fall further behind. There does reach a point where you won't be able to take another subject because you would be too far behind.
Timetables here work fortnightly, each subject you take being allocated a certain number of lessons fortnightly (9 periods/hours a fortnight per subject. There are 5 periods a day across a total of 10 days, so there are 50 periods in total). As we're also a school, we also have PSHE and PE/Games, PSHE taking up 2 periods a fortnight and PE/Games taking up 4. In year 13, Games is optional. So that's 6 periods from the 50 periods, leaving 44 periods left. Therefore a maximum of 4 "full" A-levels can be taken (9*4 = 36 periods). However, there's also Further Mathematics which takes 5 periods a fortnight, so you can take a maximum of 5 within school timetable (at least at my Sixth Form).
If you have 3 A-levels, you have 44-(9*3) = 17 periods remaining
4 A-levels (inc. Further Maths) give 12 periods remaining.
4 A-levels (excluding Further Maths) gives 8 periods remaining.
5 A-levels (inc. Further Maths) gives 3 periods remaining.
The remaining periods are a mixture of Study Periods and Free Periods. I personally have never had a study period as the lowest I've taken was 4 A-levels (exc. Further Maths), so I can only give second-hand information about those. As the year progressed, depending on performance, Frees were either replaced by Study periods, or were given in place of Study periods. Study periods are basically sitting in a room with others doing work (I believe you have computer and mobile phone access) which is partially supervised and registered. Free periods you can do what you want, for example going home, or staying in the common room/library or the like.
I think some Sixth Forms will allow you to come in whenever your first lesson starts (i.e. stay at home when you have frees), though this might only be a College thing. Here, you have to be in like you would if you weren't at a sixth form, and register (with a teacher) as normal. We also aren't allowed to leave school grounds before lunchtime, though it doesn't stop it from happening (typically when people have no lessons from period 3 onwards, they'll disregard the rules and leave). I haven't seen anyone get punished for this so far, though it's at threat of punishment (I believe removal of right to leave during frees). We also have to sign in/register (not with a teacher/staff) during pre-lunch free periods.
Homework is set like a normal school. It depends on your teachers. Personally I find that some teachers basically never give out homework and then start feeling bad about it, giving out homework for a few weeks and then digressing back to not doing so. Others hand out homework consistently (or inconsistently).
Lessons can get cancelled, which effectively gives you a free period, which is usually nice. It can get annoying if they get cancelled very frequently though, as it ends up wasting your time too -- I find it infinitely harder to do work in frees and as such have long since abandoned attempting to do work there. I do all my work either in lessons or at home, unless it's something I really need to do or really would like to get out of the way. Frees have a lot of distractions, personally.
In terms of assessments, you of course have the small class tests and whole year tests, which will have a role in informing your predicted grade should you do terrible in your Year 12 end of year exams/mocks/UCAS exams/AS exams. We don't do AS exams here. For subjects that typically large number of students participate in (e.g. Mathematics), the proportions of students that get specific grades in the school is relatively constant, so students can be ranked according to their exam grade, the proportion applied and a quick logic check of whether it makes sense for students to be given those grades based on their prior work (e.g. an A student suddenly getting a C), in which other evidence will be looked at. Typically predicted grades are slightly inflated. The highest grade in an AS exam is an A, the highest in a UCAS/mock exams (the exams are made by the school, not UCAS) depends on school and subject. We've had a lot with A* being the highest, while also others that have A like an AS. Predicted grades of course go up to an A*.
You sit A-levels at the end of Year 13. You apply for UCAS universities in October of Year 13 if early entry (e.g. Medicine/Dentistry, Oxbridge), and a couple months more if normal entry. School internal deadlines may be different.
Feel free to ask if you have any specific questions.