I can only speak for myself, but if only have access to the Galleries lounge due to guest numbers, I will frequently use T5B, even when I know I'll be leaving from A, e.g. a domestic flight. It doesn't take long to get back.

You just need to use the passageways, there is a great guide here: https://www.flyertalk.com/forum/brit...assengers.html

Don't try to get back to A via the transit as I'm sure you know. It will not end well