As the success rate for both schools or GS entry is similar, and the state school is rated Outstanding, I would be inclined to save my money. That is unless your fallback option is an independent school in which case the CE preparation will be very valuable.
I too wouldn't worry about the immigrant children - many immigrant families and their children have a superior work ethos to British children, and can actually raise standards in their schools.
I guess I can also teach VR and NVR at home as there are the Bond Papers to use.
The change from Moray House papers to NFER also changes the type of practice material you need to use. We do not recommend Bond for NFER VR preparation. I haven't been following the Warwickshire changes in detail for the last couple of months, but please do go to the Warks section of the Forum and read the posts there so that you can make sure you are using the correct preparation materials.