Ok so if you are thinking Walton or Weybridge, there are the two Heathside schools (Three Rivers in Hersham does not have a great reputation but that might be unfair. It is the old Rydens in a great new building.)
There is a section on the Surrey schools admissions website on applying from abroad. Do you have an address in Surrey - it also gives details of the evidence you need to supply esp if you don't have a property, and says you should still apply by 31st October.
If you move before 8 December then you could update your address and still be on time application. Otherwise will be a late application so dealt with after the ontime applications, thereafter you will be on waiting lists (which are kept in order of the admissions criteria for each school, not when you applied).
So a couple of things really:
Read the Surrey admissions website so you understand process and dates, and follow this
Read each secondary's admissions criteria and allocations of places in previous years (also on Surrey website) to understand what actually happens - eg if they fill all their places from feeder school pupils
Move as soon as you can, and if distance is a criteria, as close as you can to preferred school
Cross your fingers and hope for March, and thereafter that waiting lists move in your favour.
You WILL get a Surrey secondary school place somewhere. The stats show that last year, the undersubscribed schools were Three Rivers, Epsom & Ewell, Ash Manor, Christ's College, Kings College, The Priory, Therfield, The Beacon, Carrington, Merstham Park, Oakwood, Magna Carta, Sunbury Manor, Thomas Knyvett - all made offers to late applications or centrally offered (ie wasn't listed as a preference). They might not be your ideal schools but your child would have a place and can go on waiting lists elsewhere.