Even on a "good day" this lounge is only open to Priority Pass members until 3:30, but for the past two weeks they won't let a Priority Pass member in AT ALL.
They have a sign up stating that it is exclusively for British Airways customers.
Both days that this happened to me it was on a Sunday, but according to the website it's still supposed to be accessible until 3:30. Both days I was there at around 12:00.