As has been said earlier getting permission to work to commence, the relevant approvals and security clearances for the contractors can take time.

The entrance to the lounge is in a relatively tight corridor leading to the gates and I would imagine that there would need to be a plan to mitigate disruption to the pax walking to their gate.

Added into this I would imagine that the Insurance company may take their time to settle which would be a hold up to work starting.

I know it can be easy to point the finger at BA (something I'm not immune to) but I would imagine there's a lot here outwith their control.