I can't give you an authoritative answer, but I can tell you that every non-Ryanair flight I've been on (OK, GR, RC) has been from a satellite.

I am saying "A" satellite because there are two - and I'm not sure if you can get from one to the other. In practice it should be possible to get from Satellite 1 to Satellite 2 (but not the other way round) by simply walking down the stairs and getting back on the train. The train goes on a circular route from the main terminal (departures) to Satellite 1 departures to Satellite 2 departures to Satellite 2 arrivals to Satellite 1 arrivals to the arrivals section of the main terminal.

AFAIK the Servisair lounge is at Satellite 1 (it was last time I looked). There is/was also a lounge at Satellite 2 (which I used when departing on OK in 2007), I don't know who runs it.

The Stansted web site implies that the lounge is accessible to all passengers except those flying on FR and on Easyjet domestic flights