The spread is always wrapped around the underlying market spread.
So if the ES spread is 1200.25/1200.50, the spread bet firm might quote 1200.2/1200.6, but they wont quote an inside spread ie 1200.3/1200.4 .
Actually worldspreads claim zero spreads, and spreads in general are getting tighter and tighter so maybe one day SB spreads will be smaller than the underlying market spread... buts its unlikely without the SB firm resorting to dirty tricks like requotes and rejected orders.