We send DD to a prep school - our friends send their children to a combination of state and independent, and many family members are teachers in state schools.
These are the factors we have taken into account and what we've learned:
DD is an only child, we will only have to pay one child's education - this made it feasible.
We have a combined income of just over £200K - but fairly low outgoings other than school (low for London, not low-low).
We wanted her to be in a smaller class - BUT there is a downside to this, even in Reception and Year 1 she is acutely aware of her 'position' in the class - i.e. she knows who's the best reader, and who's best at maths etc... this can be quite stressful for her, even at such a young age.
They push their kids to perform, her reading and maths may be in the middle of her class of 12, but would be above a standard Yr1 in state school. This isn't because the children are more able, it's because they are hurried and pushed to learn at speed. This may not suit all children, I'm not sure if it suits DD yet.
The parents at DD's school are largely high-income, middle class, people - so have jobs in finance, medicine, law or accountancy. But work full time/long hours are fairly 'normal' (no sprawling mansions, Bentleys, or live-in staff).
Another reason to choose an independent school was access to facilities, and having a school that isn't struggling for money for equipment, building upkeep, etc.
There is a well-liked state primary nearer our house, we decided against it for the class size/facilities reason.
Bottom line: we want to give DD as many opportunities in life as we can afford, DH and I are both state-educated, and to be honest, didn't reach our full potentials in education.