The Surbiton ticket is valid as the Freedom Pass is counted as a Travelcard Season (ie, a Season and a Zonal ticket) however you may have problems with the Surbiton ticket, as some may think the train should call at the station. Buying a Boundary Zone 6 ticket avoids that issue.
If buying online is easier, I'd be tempted to email SWT asking them why BZ6 tickets are not sold online, are there any plans to change this, and does it make a difference which ticket is purchased? You could then bring their response in case you are challenged.