Hi. Arriving on CX in Premium Economy around 7am, departing some 16 hours hours later on BA in J, separate PNR, OWE.

I'm going to get a day room at the Regal, but wondering whether I can get to a lounge for breakfast before I check into the room?

My BA BP will get me in I'm sure, but

a) can I leave the airport once I've gone through transfers and back to the terminal and had my breakfast, and

b) undecided about luggage. HBO I guess it doesn't affect anything, but if I check it in I guess I'll have to clear immigration, collect bag, then either check it in again (will BA desks even be open/willing to accept bags this early?) Or would it be a matter of leaving the bag at the hotel and walking back to the terminal and going through security etc?

Or is all of it just too much hassle and I should spend a few bucks more by eating landslide in the terminal or hotel?!

Thanks in advance