I have searched for days hoping for help but have not yet located an answer.

Please help with a question: we are flying BA F from ORD, connecting to BA Club Europe to FCO with an 8 hour layover at LHR. Please explain what lounges we can use during that time since our class of service will change. I have figured out that we can use the Arrivals lounge at beginning but our FCO flight leaves at 18:45 and the arrivals lounge closes much earlier. Can we move into a F lounge from there even with the CW outbound?