The tattler good school guide. It's the best guide imo for independent prep schools.
Read it thoroughly and start narrowing down from there based on what you think resonates with you.
For me it was freedom away from the national curriculum (tired, out dated, not fit for purpose) and a emphasis that wasn't based on academic achievement but the development of the whole child. A family approach where mud and the English weather were embraced and children could just be children. Snowball fights, classes that looked and sounded fun, an environment we're learning for learnings sake is encouraged.
The first time I met the head of my eldest's prep school, he was dressed as Just William for World Book Day. As we chatted, the school dog trotted round the corner and curled up infront of the open fire. Frankly, I was sold then 😆 But your priorities might be utterly different.
At another later school (we've moved a lot) there was a wooden sword fighting after school club. It was glorious.
On a serious note, I also want teachers who are happy and well paid (I will and do ask if they have opted out of the teachers pension scheme and what they provide instead if so as this affects recruitment). Fulfilled, well qualified teachers, some of whom stay for ages and some who move onwards and upwards to leadership positions.
I also find out what the food and the loos are like. Do they outsource their food to a for profit company? Why? Try and eat lunch on a normal day there and not a special open day where the food will be better.
Basically, I just wanted a happy child because happy children thrive.
Finally; there's no "perfect school". You just pick the best you can with all the information you have and if it doesn't prove right for you, there's other options. There's always other options in the Home Counties.