Colleges. Hm. "may offer A Levels but not as many". My college offers 17 different A Levels, the 6th form of the school I went to offers less than that, definitely.
My college offers courses ranging from level 1 (below GCSE) to a degree level. I chose to go to a college because they give you more independence at A Level. I didn't want to feel like I was in a secondary school for another 2 years of my life (I was seriously over it). Yes, 6th forms are more strict, but college is like a stepping stone and alleviates the shock of moving from a very strict environment at a 6th form, to life at a uni.
Colleges, 6th forms, or even 6th form colleges are not preferred over each other by unis. It's about the results you get.