Eversince the LHR-KIX route was cancelled (well, "suspended" they called it ) they have used the UA lounges for their passengers. Access is as follows;

United First International Lounge: ANA F passengers, ANA Super Flyers / Platinum members
United Red Carpet Club: ANA C passengers, Other Star carriers' Gold members

For your reference, the RCC is really nothing to write home about. Given that you also have access to the other Star lounges in LHR T3, I would say either AC or SQ lounges will be far better. (Now I don't even bother going to the RCC anymore)