How many flights are cancelled will depend on who exactly walks out and when. It’s entirely possible to fly on some routes without interacting with a single staff member - OLCI plus SSBD plus self-boarding gate. Have a few managers to sort out the issues and things could move as normal. (Obviously they won’t in practice, and delays will be rife, but I doubt BA would just throw in the towel and cancel everything. It’s not as if the staff concerned are as critical as pilots, for example.)