BA flights arrive at the non-Schengen end of the international pier. From here you'll need to clear two immigration checks: the first shortly after you disembark and the second to get back into the non-Schengen departures area.

For the first immigration check, you should ideally head for the desk marked 'international transfers', which is to the left of the arrivals immigration hall. The only advantage of using this desk is there's usually no queue.

NB The international transfers desk isn't always open, leaving you with two options. The first is to ring the bell at the police office at the back of the main immigration hall and ask if they could open the desk. The second is to join the main immigration hall and be prepared to queue-jump if your arrival co-incides with a long haul arrival.

However, once through the first immigration point you're filtered back into the arrivals corridor which everyone else uses (i.e. it's not a non-Schengen to non-Schengen path). About halfway down this corridor, look out for a door to your left marked 'Transfers'. Here you'll re-clear security. It's then down a staircase into the non-Schengen departures area and a (recommended) brisk walk to the far end where you'll re-clear non-Schengen immigration.