Guess I'll just go to New Cross Gate and buy a single there - at that
point it's only £8.60 - a bargain.

One-TOC fares on that route are cheap, I agree. Privatisation working
well for a change. In this case it'll be "Southern Only" from New
Cross Gate.

Plus £2 to New Cross Gate of course

If we are looking outside the peak periods then I think the cost could
be shaved a bit further at the price of a change at East Croydon: Oyster
to East Croydon (£2.80) from where it's £4.90 to Gatwick - total £7.70.
--
Robin
reply to address is (meant to be) valid