"Wilshaw and Sutton have said comps don't cater well for bright kids so frankly the fact that a few do( often in places a lot of families are excluded from ) doesn't make it a successful system imvho"
It shows that it is possible, with the right resources.
The bottom line is that schools are communities where children's learning is influenced by each other as well as by parents, teachers, etc.
Our school system currently - by and large - is one of social apartheid. Children are pretty much divided into schools along social lines. If you remove about 25% of the highest achieving and best supported children into faith, grammar and private schools, and create a system of school selection in comprehensives which favours the sharp elbowed m/c, you are going to end up with educational ghettoes, and that's what successive governments have done.
They have deliberately created a system which clusters the poorest and the richest children into separate schools. Poor children are only allowed into the 'rich club' if they have something special to offer - that they are clever or talented, otherwise they are excluded.
Some of us, particularly in London, have our children in comprehensives which are genuinely socially mixed and mixed in terms of ability. My dc's school has low ability children from some of the poorest estates in London mixing with the children of doctors and lawyers. And with good resources, management and teaching and a genuinely comprehensive intake these schools can and do work.
But yes - create social and educational ghettoes and then point at the ones containing most of the poor and disadvantaged children and say, that's not an environment in which a high achieving child can easily thrive, and actually there is some truth in that. Children don't thrive well in deprived communities generally. But don't blame the schools - blame the government for creating this system of social apartheid in education, and then blame yourself for advocating for things which deepen the divide and perpetuate the inequality - like academically selective schools.