Originally Posted by
aglp2k
Quick question regarding access to T3 airside. I will be arriving into and leaving from T5 on BA next month and have few hours in transit. Is it possible to visit a friend in T3 airside during the layover?
Yes it is. T1 and T2 check boarding passes, but T3 isn't doing so. You can't use the T3 BA lounge, at least in theory, even as a guest, unless you somehow sweet talk your way in. But you can use the Pay Per Entry lounges, indeed they are now advertised as available in T5 bus Flight Connections.
To do this, after arriving into T5, follow the signs for Flight Connections All Terminals, then in the main hub area Flight Connections T3, which involves an escalator down to the buses. Do not go through T5 Flight Connections first, since it's troublesome to get out of airside. At T5B (not C) there is a direct bus link to T3 down one level from arrivals, one level up from the transits.
For the same reason - and for the benefit of anyone else searching here in the future - you cannot do this if entering T5 from landside, this only works if you are on a connection, and a non Domestic-Domestic one at that.