I went to SSH in J on a 772 a few years ago with SHMBO, very good BA Holiday deal for 5 nights and using MFU as it was in those days (flight was very empty in all classes both ways - two of us plus the Captains family in J coming home).
One word of warning, though it may well change, the 'lounge' was up an escalator and is literally a dark room with views of the apron. No TV no wifi, no papers, nothing. There was a table with several cans of soft drinks and some packets of crisps, nothing more, nothing less.
At check the guy in front of me was kicking up a fuss as he was told that lounge access was for passengers in CW only and not extended to G/SCH. As I was both I didn't get to ascertain on which basis I was admitted to the 'room' but we soon retreated downstairs to the bar.
Given the night flight outbound it was worth UUA in those days but for a LGW 320? nah. I'm going to FAO next week with Monarch - I'm not sure it can be any different than a GAT* 320 for 2 hours.