a) AC is indeed airside at LHR T3, I think the issue may be how you get your luggage through security to the AC !
b) FYI, once at the AC, you can quite easily go back through security and out, not a problem.
c) Each terminal at LHR has a baggage storage location outside baggage claim....some not too well signposted in the terminal, best to go the the BAA (British Airports Authority) web site and check the terminal map for T3 LHR.