Looks to me like it is the address at the date of the application via the Pan London form (? October)... this is the info from the Wallington site :
The address to be used for the initial allocation of places to Year 7 will be the child’s address at the
closing date for applications within the Pan-London Co-ordinated Secondary Admissions Scheme.
Changes of address up to a date in December determined by the Pan-London Co-ordinated
Secondary Admissions Scheme may be considered if there are exceptional reasons behind the
change, such as if a family has just moved to the area, or has returned from abroad. The address to
be used for waiting lists, after the initial allocation, will be the child’s current address. Any offer is
conditional upon the child living at the appropriate address on the relevant date. Parents have a
responsibility to notify their Home Local Authority of any change of address.
Am sure someone will have even more idea than me (more evidence the better)!!