For Jan 2018 the entrance exam is on same day as Kingston Grammar therefore encouraging parents to make choices. There is a back up date too. The number of spaces depends on number of children feeding from Junior School. It's a good school but you can't rely on getting a place.
If you visit St George's you should fit in visits to Sir William Perkins (known as SWIPS), Norte Dame school for Girls and St John's Leatherhead with possibly St Teresa Effingham or Surbiton High - this range of schools will help you to gauge the ability level needed to be offered a place. If you want to stick with IB try ACS Cobham too! For additional mixed schools with varying levels of entrance difficulty compare St George's to Kingston Grammar, Epsom College and Claremont Fan Court School.
St George’s have traditionally taken a third from their junior school, a third from prep schools and a third from primary schools - no quotas as such, but that’s how it usually works out. They have exactly 110 places in Y7/8.
The entrance exam is VR/NVR and a maths paper, iirc.