I've been successful when Silver in LGW Terraces - which seems to be quite common. Perhaps you should compromise and try for the Galleries Club rather than the Galleries First?
No more recent experience as recent trips we've used the lounge for our travel class.
In my experience LGW is a lot more laid back, especially as most users of the FLounge are Gold rather than First. But does depend how busy.
I think the Club Galleries might be an easier nut to crack at T5.