The T5B lounge is my go-to when I'm at LHR. I'm not aware of any long-haul flights that depart from T5A, except for those by bus/coach. I generally head straight there after security at T5, usually walk through the tunnel to get a little exercise. The agents there seem more happy and helpful, the food looks fresher, and I've never had to wait for a shower there.

So does BA give lounge access to all elites and/or everyone who buys Club Europe? I've wondered why the lounges in T5A are always so busy (occasionally I'm with others, or I don't know my gate, so I'll stop there). Perhaps BA needs to limit access, similar to how AA does for most USA domestic flights. Even if I buy a premium cabin, I generally won't get lounge access domestically in the USA, unless I buy it (membership or daypass), or am connecting for travel outside of the US (there are a few exceptions where I will).