Note that FCC no longer exists.

The blanket statement "it will be £15.80" isn't correct. While all Travelcards purchased as a combined ticket from Gatwick Airport will include zones 1-6, the price ranges from £10 to £63 depending on the day and possibly time of travel, class, and which train companies you wish to use.

Outbound on Saturday a "Thameslink Only" travelcard costs £10 from Gatwick Airport ticket office (there's only one ticket office and it sells all tickets). Then you get a train to Blackfriars and the District line to Earls Court. Despite the name, the ticket is valid on all services within the zones. The "Thameslink Only" part means you need to use Thameslink trains for the part of the journey outside the zones. The ticket is in fact valid for a journey back to Gatwick that day but you just don't bother to use that bit.

For Sunday get a zone 1-6 Travelcard from any station for £8.90. Don't bother with Visit Britain. You cannot get fewer than zones 1-6 for a lower price, as they were withdrawn from sale a while back (unless Visit Britain somehow still has some ancient stock left).

Cheapest option on Monday is a Thameslink Only travelcard again, from Gatwick to zones 1-6, costing £13.50 this time. This is slightly cheaper than buying a single ticket on the tube to Blackfriars for £4.70 and a single ticket for the train at £10. But you have to buy the train ticket from a train station and not a tube station.

You can buy all three of the above tickets from Gatwick Airport station on Saturday if you wish. Unlike the Netherlands, just about all train ticket vending machines take credit and debit cards, and most take banknotes and coins (pounds sterling). Ticket offices also take all of the above and there is no surcharge for using a ticket office or a credit card. At Gatwick Airport ticket office euro banknotes are accepted too, but there's a very bad exchange rate. However, V-Pay and foreign Maestro cards aren't accepted.

You can also buy them online but it's a bit of a pain.