Originally Posted by
UKtravelbear
The details of your transit matter.
If you went through security you must have changed terminals
Certainly arriving into T5 there is no security from domestic into the departures area. Though I'm not sure about T2- the only other LHR terminal with UK domestic flights.
At T2 if the staff set the doors correctly they can route domestic arrivals passengers to departures without rescreening. The route isn't always open as it has to be staffed and can often be dead.