Never flown BA before, but with my experience flying ZH , the lounge will stay open until the last possible departure for the day. I remember my 2017 Summer WUX-CAN ZH J flight for 150 USD scheduled to depart at 8:30 PM, announced no EDT delay, with the aircraft scheduled to fly our flight still sitting on the ground at CAN, the lounge staff told me that their shift starts at 5 pm and will end after the last possible departure for the day, the aircraft only left CAN at around 1 AM and we eventually departed around 3:30 AM, is good that ZH offer CD even for discounted J passengers back then. Ended up filling my insurance and got compensated 100 USD per person.