Originally Posted by BasilBush

Connecting off a domestic flight at LGW is a bit of a lottery. If you are lucky to arrive on a proper domestic gate you simply head for Flight Connections and you will not need to reclear security. However you will have to have your photo taken.

However, if (as often happens) the arriving plane parks at an international stand - generally because the plane's next flight is an international one - you will endure the tedious process of being bussed. The bus will drop you at the domestic reclaim hall, from where it is not possible to access Flight Connections (although God knows why not). You will need to exit, go up two floors to departures, and reclear security.