In the fullness of time indeed BA and IB staff will be TUPE'D onto IAG contracts thus saving a great deal of cash.
Sorry, but how would 'TUPING' all your staff onto new contracts save a great deal of cash? TUPE is about protecting the terms and conditions currently enjoyed by employees, under the new employer.
Management would be keen to create a situation whereby TUPE won't apply, but that current contracts cannot be maintained for whatever reason, therefore creating the opportunity of forcing all staff onto inferior terms and conditions. It would be a difficult set of circumstances to create and see off, but certainly possible.