Originally Posted by
NWIFlyer
It's not a route I travel at all frequently, so I'll happily bow to your knowledge!
I do get the impression from the National Rail website that there must be a very, very limited number of services that offer advance tickets though because only the Gatwick Express came up and that's been my experience in the past as well. For the OP's benefit, are there particular times of the day when advance tickets are available? If that's the case, if those times are no-where near the flight arrival, the discussion and options certainly narrow a bit!
Sorry if I came across a bit brusque.
First off, the OP wants to make use of the '2for1' offers available with a rail ticket for her stay. Given that, she should buy a return (round trip) ticket as this means she can take advantage of the 2for1 offers throughout her stay in London - Advance tickets would only provide for 2for1 discounts on the specific days they were valid for travel. It's explained
here.
However, if one did want to get an overview of Advance tickets on that or any other route I'd suggest using one of the ticket booking sites based on the "WebTIS" platform (with the 'mixing deck'), such as
GWR or
Chiltern. (And specifically search for journeys between Gatwick and London Victoria, rather than just 'London'.)