yes it is nice (KUT), don't write it off. I don't know how old your children are, but if you have time, I'd wait and see what happens with this new school (if it does happen).
we moved from zone 2 to 6 and even that took some adjusting. I don't think we will ever move further out - in fact any subsequent moves will probably take us further back in.
The Tiffin thing is a red herring - don't forget Kingston also has Sutton fairly near with its grammar schools - those schools don't only attract children from the local area.
In RUT, a lot of the secondary applications are still done by link schools - as gegs said - so you may need to get your child into the last year of primary school at a linked primary if you want to be sure of a place. This has become more important over the last couple of years as certain primaries have started sending larger proportions to the linked schools.