Yes, you can use it if connecting at T5 [with the exception of UK Domestic to Rest of the World connections, in that direction only], though goodness knows, it seems a bit of a flap to do it. The reason being that access to the smokers' cage in T3 is available immediately after leaving the bus and also when departing. So arriving in T5 you follow the signs to T3, take the bus over, at the bus stance follow the clear signs to the Smoking Area, have a puff or two, then follow the signs back to Flight Connections T5 and the bus back, which actually leaves on the other side of the smokers' cage. The reason this is possible is that there are no checks on boarding passes for using the bus, and the Smoking Area is before security at T3.