Well it's possible. I figured broadcasting content before the launch might require coordination between the operator and the station, and as not many people would be listening before the official launch, maybe they wouldn't go to the hassle.
The original Virgin launch was probably more of a big deal, and there would have been more incentive to tease the audience running up to the launch perhaps.
Presumably the test stream does not play out from Virgin's studios at the moment, and the mux operator plays out the stream? I guess if they did want to have prelaunch content, they'd have to send Sound Digital new test streams or switch on the link before the official date?