Another vote for T5B, as long as you’re confident about using the walk ways to get back to T5A.

There was a 20 minute spell last Feb where I had the entire place to myself. Although some reports on here recently have suggested there are times when even the T5B lounge gets fairly busy.

Failing that my chosen spot would in Galleries South. On entering turn right and walk as far as you can until you reach the glass wall. There’s a bar area and coffee machine in that corner. Pre-covid I found you were almost always guaranteed for that to be a quiet corner. Post covid, after about 10-11am, even this spot can start to fill up.