There are 2 different situations here.

1. You decide to go landside and transfer terminals / go into London on a connection.

For this, at T5 (just like every other terminal) you follow the signs for Arrivals and Immigration, go through immigration and customs, just like a passenger ending their journey in London.

2. You decide you want to leave the departures area after either (i) going through security or (ii) going through Flight Connections.

For this, at T5 they have to call a member of BAA staff, who may take a while to arrive, to escort you through into Arrivals, and from there to the landside area. Once you are on the departure level of T5, this is the only way they are allowed to get you out of the terminal. The reason for this is legal, not architectural, and is because UK law requires arriving and departing passengers to be kept completely apart.