Flew LGW/BCN/LGW at beginning of July. Check in is in Terminal B and security there (directly up the stairs next to BA check in) was a very quick affair. Unfortunately, we did exactly what you describe. We were told to make our way to the gate which was beyond border control to find that the lounge there was not BA / IB. The security staff took pity on us and allowed us to re enter the main terminal. So once you're through secuity you need to turn left , I remember travelators along the windows, and somewhere there on your lefthand side you should be able to find the entarnce to the IB lounge. Entrance is pretty gloomy if I remeber correctly - the door is tucked away in a corner. Oneworld list is displayed inside. Lounge was decent size, very red in colour and very basic in what's available. It is then a lengthy hike to the A gates to board when he BA flight is called.