Architecture and security policies get in the way of OP's access request at LHR. But, the general solution was for the agent to have issued OP the requested one-time pass. That would only have worked at a UC and not any other *A facility. Alternatively, an "invitation" coupon for Arrivals (which could also be entered in the PNR notes).

OP should be able to credit his BA flight to any OW program and to claim ORC from UA.