Sorry to jump in on this, but how exactly do you buy a travelcard online? What do you put in the journey planner? Happy to use the railforums ticketing site!
As a general rule, to buy a zones 1-6 Day Travelcard search for a return journey from a zone 6 (or zone 5) station to the London terminal - a Travelcard should be one of the tickets offered (but do double check that's the ticket you select).
(Whilst a Travelcard is valid regardless of the purported origin station printed on it, I might suggest choosing an 'appropriate' origin for the area where you will start using the Travelcard to avoid any potential furrowed brows of ticket inspectors.)
Some example journey searches...
Surbiton (zone 6) to Waterloo
East Croydon (zone 5) to London Bridge
Orpington (zone 6) to London Bridge
Bromley South (zone 5) to Victoria
Upminster (zone 6) to Fenchurch Street
Romford (zone 6) to Liverpool Street
Enfield Lock (zone 6) to Liverpool Street
Crews Hill
or Hadley Wood (both zone 6) to Kings Cross
or Moorgate
Elsree & Borehamwood (zone 6) to St Pancras
Harrow & Wealdstone (zone 5) to Euston
Harrow on the Hill (zone 5) to Marylebone
West Ruislip (zone 6) to Marylebone
West Drayton (zone 6) to Paddington
Heathrow Rail (zone 6) to Paddington
As
@signed states above, some ticketing websites also offer other (simpler) ways to buy one - plus it's worth noting that whilst the
Southeastern site is mentioned, the same interface/system is also used on the
Southern,
Thameslink and
Great Northern sites too.