The ERJ135 is to become G-SAJB when delivered in October, whilst the ERJ145 will become G-SAJC in November, all according to Jethro's. I imagine they did not pick the all-white ones since the LM examples will be repainted in full Tartan rather than vinyls like the S2000s.
I'm guessing the BA style acronym of the reg is perhaps G- Scotland's Airline Jet X ?