There is, but the security guards will rarely let you use it. I was permitted to once, and only because I had a tight connection (DFW-LHR-DEL), and the flight to DEL was about to board, or even boarding by then. Otherwise, they make you go to the main security in T5A.
A friend of mine sweet talked her way into it once also, on an ATL-LHR-PVG connection. I was waiting for her in the T5B lounge, our flight to PVG had a comfortable cushion, and they were trying to get her to go to T5A, but she showed them a text of me telling her where I was and they let her use that checkpoint.