SWT Map > http://www.southwesttrains.co.uk/networkmap.aspx
TfL Zones Map > http://content.tfl.gov.uk/london-rail-and-tube-services-map.pdf
So Kingston is in Oyster TfL area,
so I assume that you can swipe in to Kingston using an Oyster and also a Contactless Debit Card, but this is only for journeys with TfL Zones,
Guildford is outside,
IF they have Swipe card readers at Guildford I assume they would only read SWT Smart Card system
but I would of thought they wouldn't let you out, unless they do and thev charged you a not tapped in/out fee in same way Oyster does,
this is normally quite high and in excess of any amount you would of normally paid.
There are two options here, you know the area well and were trying it on to avoid paying the full fare.
You dont know area well and the combination of Oyster/SWT SmartCard system/Contactless debit card system has caused confusion,
I know Im bloody confused and Ive spent a while looking into it,
further confusion is added by the fact that some TOCS (such as my local ones GA and C2C) usem same fare structure as Oyster
BUT some others use a different higher fare structure although you are still within the TfL Osyer area.
If its a genuine mistake on your part then Transport Focus have been useful in persuading TOCs that prosecution is not the best option
Also see > http://www.transportfocus.org.uk/research/publications/ticket-to-ride-an-update