The range of standard class advance fares from Swansea to London Terminals goes from £12.50 up to £102.50. There are no fewer than ten tiers of standard advance fare on the flow, and there is a 50p addition if travel on ATW is contemplated.
The lowest tier of Advance fare will not always be available on every service; it will depend on what FGW thinks the market will bear and on how many people have booked seats on the trains so far.
I recently got Advance tickets at the lowest tier for a Friday evening train from Swansea to Paddington, so they definitely still exist! Date of travel is late October, so it may be that you have to be pretty quick to get the low fares.
I've never been able to get any within a couple of weeks of travel, I usually buy a walk up ticket as the savings with Advances can be minimal (or non existent).