This thread is about how "effective" it is to spend tens or hundreds of thousands of pounds on children's education at different points of their lives and contains huge misunderstanding and misconceptions.
For example, the claim that "the bar is low" in state primaries. No it isn't, it is just measuring something different to private primaries, which are measuring one thing only, the ability answer abstract questions in selection tests at 11 or 13- no one needs to know the answers to these questions at any other point in their lives, and concentrating on these actually means that children from private primaries are often going to be classed as behind if they transfer to state.
Nannies are talked about as if they can provide something extra educationally during infancy - no they can't. I was a nanny myself for over a decade, and moved within nanny circles in London in that time, and knew literally hundreds of nannies.
A good nanny provides a high quality of child care, safe, and fun. They cannot provide anything developmentally that a parent cannot provide better. Simply because they don't have the same relationship with the child. Parents have a long term relationship with the child based on love, nannies have a short term relationship based on money, and children know that - at lease they should know that, it is very harmful if they don't.
Therefore, the best "investment" in your child's education in preschool years is your time, which may well involve less hours working and more sacrifice of salary. No amount of money can pay for anything equivalent to quality time with parent.
Other comments, such as saying a state primary "will do" and you need 5 figures a year to pay for music lessons are just snobbery and ignorance.
Privately educated people do not "do better" than state educated people in the long term, on average - some people have a good experience in private, some people have a terrible experience - same as state. The same teachers teach in both, at different times in their careers, the teaching is the same. Private schools often go for the cheaper and easier iGCSEs, which state schools are no longer allowed to offer (because they are easier - keep in mind they are designed for poor schools overseas with no resources, no internet, no practical equipment, no electricity, no photocopier etc - private schools in the UK are not what they were designed for, but such schools have adopted them because they are cheaper to run and easier to get high grades in)
Keep this in mind when comparing private and state school outcomes - that most private schools are using the easier exams..
The teaching is the same, the exams are probably easier - some people do better than they would in state, some do worse
There is none of the benefits that people imagine there are - your child likes their school and is happy there - great! pay if you want to- but also be clear about why they are happier and how they are going to manage once they are out in the "real world"- they are probably happier because they are sheltered, and have access to more facilities..
It isn't to do with better educational achievements.