Background info: I'll be arriving at LHR from an international flight at T4 at about 6:30am, and I bought a separate BA UK domestic ticket departing from T5 at about 5pm in case of flight delays but I have the option to change to an earlier flight on the same route on the day itself for free.

Assuming I do not change my BA flight ticket (due to delays etc.), what is the earliest time I can check in my luggage for my BA UK domestic flight? I am aware we can check in online up to 24 hours before flight. Can I check in my luggage as soon as I've received my boarding pass? Also, after I've checked in my luggage, can I travel from T5 to T3 airside to use the lounge there?

Thanks for clarifying!