If that were the issue (BA aircraft, managers, capital = BA crew), you would be fighting to get Cityflyer crew on the master seniority list as well; I do not hear any mention of that...
Perhaps it would be a good time to stick up for your colleagues there as well...make a change from when existing crew did not stick up for new crew having to join the DC pension scheme.
The very fact that this new operation, operating new routes from bases with lower costs of living, in new markets, free of the bureaucracy of the existing operation, when the legacy routes are to continue operating from London with existing crews safe, appears to me to be an entirely new operation - who cares what link it has with BA, so long as you prevent crew and aircraft from the new business operating from the existing bases: ever.
Since there is a precedent with CitiExpress operating as a fully-owned subsidiary outside of the master seniority list, I fear you would lose any arbitration or court action if BA were to hardball with the union.
Not to mention that you would tie their hands if they ever wanted to sell off this new operation - if unsuccessful - as it would be too intrinsically tied to mainline.