Lemoning: sorry I didn't reply sooner, I had gone to bed! For the record
It is worth it to us for several reasons:
my daughter is very bright (sorry - I know everyone thinks their children are bright, but she is. Not my opinion - the opinion of every teacher who has ever taught her) and she has been extended far more in private school than she was in state school. This is helped by the fact that the school are not straightjacketed by the National Curriculum and tests.
My youngest son has ADHD type difficulties - he struggled badly in a state class of 31 children, but his current class have 16 children and a LOT of structure, and he is thriving.
There is a far greater selection of extra curricula activities on offer than at State school, which I think is extremely important. The school they go to also spends a lot of time "performing" - the children are involved in at least two drama shows a year, a well as dancing and music etc. I think this helps their confidence so much. They also do a lot of "just for the school" performing - for example children are offered the chance to play the piano etc in assembly. They just really celebrate all sorts of achievement.
Their particular school has a much more diverse cohort than the local -all white/pretty much all born in the village/all Protestant church going- school. For me this is important as it broadens horizons, and helps children to be aware that there is a big world out there full of people who do things different ways, and who move around the world rather than living and dying in the same village...
Also the slightly extended school day is useful to me as it means I can often do pick ups and drop offs without needing to arrange additional childcare.
In the end though, the most important thing I think has been the small class sizes - it really means that the teacher gets more of a chance to interact with each child meaningfully, and without being pulled away by 29 other children. Whatever the child's needs (and even if their needs are decidedly "ordinary") this has to be a good thing.