Bypass immigration at T5?
Recently I flew from CPT to TXL, taking the airside transfer bus from T3 to T5. As expected, at no time did I go though immigration and passport control in LHR, only in TXL.
However, I noted that I could easily have boarded one of the domestic flights from Gates A1-A6 (on a different ticket?) once I was in the departures area of T5. Is there separate immigration and passport control in EDI or GLA, or is this a way to avoid immigration control for those so inclined?