There aren't feeder nurseries for those schools. Most of those do not do entry by assessment but by registration at birth. Thomas's was previously assessment but has also been a lottery system the past two years.
You'll find that most nurseries between the Commons send children to those schools every year, and also to the local state primaries. It's all mixed.
Popular and well-regarded nurseries in the area include the Marmalade group, Alphabet Nursery, and Mousehole. All of them recommend registration at birth.