James
Having the VCC unused is the better option.
For BA it is better to under promise and then over deliver (i.e. reduce the schedule and then comprehensively fly it) then over promise and under deliver (i.e promise to fly the full schedule and then find that you can't!)
The latter would play into BASSA's hands with media reports of T5 full of stranded and upset passengers.
The current plan has all BA pax getting to their destination via a variety of routes. The VCC are there to ensure BA can do that. As the strike starts to fade away it will be a balance of adding additional services and allowing the VCC to return to their normal roles.