I got access to the posh sub-lounge a couple years back when flying Sri Lankan in Y, but with BA gold, so I assume OWE have access to the posh part (and those flying in F) and OWS (and J) access the regular area. I am not 100% on this however. I didn't have to ask or anything as I had only ever used the BA lounge at T1 but the staff directed us into the posh part, so if OWE (or not even) it can't hurt to ask when you show up if you can use it or not.