Originally Posted by
corporate-wage-slave
That's also possible, but (a) watch the opening times and (b) the JAL lounge is better located for the usual BA gates and (c) for oneworld emeralds there is a calmer but unadvertised First lounge as part of JAL (entrance on the right of reception).
The JAL Flounge is not open all day however. If you have a morning departure you are stuck with the J lounge.