Can I offer some pieces of advice here?
First, don't book with thetrainline, they charge you various fees just for the privilege of booking with them. Book direct with the train company concerned (sometimes you even get a discount for doing this), or with a site that doesn't charge, like Red Spotted Hanky (my train ticket site of choice).
Secondly, don't bother with Gatwick Express. Tickets are expensive and have hardly any benefit over the regular Southern trains from Victoria.
Also, I should clarify what someone said earlier, the cheapest tickets are called "Advances" and are always sold as Single tickets (there are several different types of "Single" ticket and just specifying a "Single" won't necessarily get you the cheapest). These tickets have zero flexibility though, and you must catch your booked train, or have to buy a new ticket. Only limited changes are allowed, namely date and time, for a £10 per ticket plus whatever the difference in price is. The usual time these tickets become available is 12 weeks, but that is not set in stone. Sometimes it might be a little later than that. The train company websites will usually tell you when you can book up until.
Also it's slightly inaccurate to say price rise. What actually happens is that the tickets are subject to quotas and the cheapest tiers sell out (which can happen extremely quickly in the case of the cheapest so book fast).
And lastly, if you are coming into Euston, there is no need to make the long slog over to Victoria for trains to Gatwick. Simply walk (or cab) less than 10 minutes up the road to St. Pancras and catch a First Capital Connect train direct to Gatwick from there.
