You should be able to use the lounge with BA status travelling Y - I have in the past. Also guesting.

It's the back of the lounge that overlooks the atrium to the bar in T3 below. They will show you to this area, but in practice you can really sit anywhere that isn't reserved.

All of the best seats with the apron views are saved for paying Ryanair passengers!

It you have PP or other lounge access cards then you might prefer the 'other' Escape lounge at T3 which is open peak season.