It is inherently not a nil-cost issue, as the ability of OpenSkies recruits to then "move up" to mainline would create new hiring requirements and training costs that would not otherwise be the case: furthermore, those recruits would join the business in London at a higher pay point than new recruits who are currently recruited directly into the London business, raising the cost for both businesses as a result.
You and I both know that BA want it separate to create a lower-cost business away from London - your negotiations with management could create the protection to prevent it operating from London airports at any time in the future - the danger to the current pilot workforce is that the London business becomes the stale rump of BA if OpenSkies is truly successful and expands greatly.
The business of BA pilots flying BA planes on a BA AOC is irrelevant - (a) as they intend to run a new AOC, and (b) as the precedents in the past of other fully-owned or outsourced operations both past and present.
Don't skirt around the issue M.Mouse / Hand Solo - the real danger to BA pilots is the new operation being a success, with no mainline pilot access.