OP, perhaps earlier posters have not been too clear that their their posts that their experiences were over 30 years ago.
Times have changed...to some extent.
The market for private sixth form education had grown hugely. Lots more students, lots more colleges.
You need to start with your child. Your hard earned money will not buy you much if the college is selling something different from the education you want to buy.
It can be useful to focus on the next stage. Where does your child want to go to University? Overseas? Perhaps look for IB. Oxbridge/Imperial/LSE/UCL/medical school look for somewhere where students routinely get A/A*s. Or is the aim a respectable course at a non RG University. Or something vocational, arty etc.
It might also be useful to look at your child. Are they academic? Are they focused on doing well. If so I might steer towards a college that appeals to parents from the far east, who will have done their homework well.
Is your child less academic, social or easily distracted. You might want to swerve places that appeal to the international rich, and instead focus on somewhere with good pastoral care. You might also decide that they would be unhappy in a fast paced classroom full of bright and highly motivated kids.
As well as the two groups I mentioned above, we have known students who have changed their minds about what they want to study so have had to take a gap year to study for an additional A level in something like maths or physics. If this had happened to my DC, MPW, along with DLD or Ashbourne, would be my first port of call. (But there will be others.) The child we know who went to MPW, from a leading private sixth form, was very impressed by the efficient teaching.