In message , at 05:46:50
on Sat, 16 Feb 2013, antoine remarked:
I think that it is non possible to estabilish before exactly
the time for the train; the airplane could be a little late, or the suit
case not soon in my hands,

Very wise.

or, or ,or. So the best thing to do is to buy a ticket for that day
but not for one particular time.

There is no discount for buying an "any train" ticket ahead of time.

My own choice (if leaving Gatwick after 9.30am) would be to get a One
Day Travelcard Off-Peak Southern-only £15.20, which will allow travel on
any direct train *except* Gatwick Express to Victoria, but *including*
the London Underground train to Chelsea, and any other bus/Underground
for the rest of the day.

Cheaper, but taking longer (change at Blackfriars) is a One Day
Travelcard Off-Peak FCC-Only £13.00, which is valid on all
bus/Underground as before, but only on FCC (First Capital Connect)
trains from Gatwick to London (via London Bridge).

If you want to be able to choose to travel on either Southern or FCC,
then the equivalent ticket to those mentioned above is £15.90, but it
still doesn't allow travel on Gatwick Express.
--
Roland Perry