These are challenging tests for young children, so you may also wish to consider boarding and country prep at this stage. 8+ is a traditional entry point and entry requirements are often much more flexible. Some schools even pride themselves on developing test that they allege cannot prepared for in advance.
Every school sets its own exam, and as such has a slightly different way of assessing. This can seem like a minefield for many parents, but it’s important to remember they are all looking for children who will suit their particular environment. Certain highly selective prep schools will insist on registration fees before scheduling a visit. If your child is accepted, these schools will likely ask you to pay a deposit, or the first term’s fees, or both.