Thanks for the advice. I don't recall any luggage lockers at the T2 lounges either (although I've never looked for them) and I'm similarly concerned with the procedures to "stop in" at Terminal 2, leave and re-enter. I just looked at Croatia's LHR schedule, and it seems like they have only one flight a day -- the evening flight. Could that actually be correct? If so, I'm guessing there would be no one at a ticket counter in the morning who could store my bags.
I suppose that if I didn't want to pay the 11 pounds per bag, I could take my bags into London city and pay less for storage. Obviously, not a great option, either.