I am a product of a very expensive private education. I had barely any friends who were state educated until very recently and it was absolutely normal in my social group for my peers to send their own children to private schools, as their families had been doing for a long time. My children started their educational career in the private sector, but after a couple of career blips and one major financial disaster, we had to take them out and we enrolled them at one of the highly rated local state schools (which we were very fortunate to get into). There were quite a number of refugees from the private sector into the school at around that time.
Initially, all seemed well as the children were very happy and found it easy to make friends. I didn't have any concerns about the workload and some of the examples of my children's work that I saw was impressive. However, as one of my children progressed into year 4, I began to have concerns over the disparity between the state and private school workloads, not to mention that even into year 6 school hours are pretty much the same as for reception at this state school. I then realised that many parents in my child's class had reserved the services of private tutors to supplement their state learning, well in advance. Indeed, I recently overheard one parent at my children's school say that she believes her child is the only child in her class not to have an extra-curricular private tutor.
The school is happy, possibly a little smug and complacent, as approximately half of the pupils go on to the private sector. There is a very active local parent body which supports the school with a huge input of time and fundraising. It could be so much better, but probably not without extra resources and maybe higher pay for teachers. It seems to do extremely well compared with national level, but some parents secretly feel that they would like the school to compete with a private school. And that is the mirage, or trap, alluded to by the original poster.
I would concur with the original poster's comments that these good state schools, however good, do not prepare children for selective entry at secondary level. Mind you, considering we are saving up to £25,000 gross income per annum per child, who are we to complain ?