Sixth Form is basically an extension of Secondary School (+2 years) except now you're studying for three A Levels, Sixth Form generally your hand is still being held by the teachers so homework and detentions still exist and extra tutoring/boosters if you need it and you're learning is more supported then that of a college education and unlike college where you can choose not to come in, in Sixth Form you'll generally still be going in everyday or your parents get notified. You also have to do mundane trivial things like parents evening or other stupid special events your school organises and you get letters/phonecalls home if you're underachieving.
But you get some privellages like Free periods where you can either study or go home or wherever, and you don't have to do things like P.E. and you can go out during break and lunch so you're not forced to stay at school or with people you hate, you also have a common room, but I rarely go there myself.
College, you have more freedom so you're treated like an adult and they expect you to do work at home, and extra reading up outside of lessons without checking up on you and if you don't do the work then they generally don't get on your case as they would if you were in Sixth Form, you either do it or fail when it comes to exams really. You also get to wear what you want and College offers more courses/qualifications other than A-Levels which is primarily academic, colleges can also offer vocational courses and BTECs, If you don't come in to college they just let it be to be honest, there are no phonecalls or letters home.