Hand Solo
The danger of a standalone sub-fleet is that IAG might decide to form a standalone airline ( "British Airways Express" ?). Being on a lower cost base they would then only recruit into this new airline and gradually transfer routes to it.
IAG would then have two disparate groups of pilots that they could play off against each other. Allowing the Bmi pilots onto the MSL with a fair, negotiated, agreement between the two pilot groups would give you a much stronger position in any future disputes with IAG. To force the Bmi pilots to join at the bottom would cause years of resentment and infighting which would play into the hands of the IAG beancounters.