In seriousness, an "EasyRail" type operator would simply have to get passengers into London, at a point where they could dump them onto the Underground. So where would be a good stopping point?
Only if it saves them money over using a terminal station.
Airports charge individually for facilities, which is why the LCCs often use remote stands or make you walk to the terminal, even when there are spare jetbridges.
At places like Geneva and Budapest, there are special LCC terminals with spartan facilities to avoid the expensive mainline ones.
Network Rail, as far as I know, does not charge for specific facilities, only vehicle type/mileage and (eg) electricity consumption.
So terminating at places like Finsbury Park or Ealing Broadway wouldn't be significantly cheaper, quite apart being just as complicated for NR as using the terminus.
The SNCF Ouigo brand is interesting in this context.
They use high-density TGVs with limited on-board facilities, with web-only booking.
They use out of town stations to turn round (eg Marne-la-Vallée), and to avoid direct competition with SNCF mainline services.
https://en.wikipedia.org/wiki/Ouigo