I find the easiest way to describe this to friends and family is that all British airport security is to the same standard so you only have to go through once on the way out. The exception being if you get the transfer bus between T5 and T3, because some of the people on that bus will have arrived from an international flight.

On the way back, everyone has come from a variety of countries and not all of them may meet whatever security standards we have set, so you always need to go back through security at LHR on the way back.

This is also the reason why Scottish travellers moan so much about security on the island flights being over zealous. You can get on the Loganair flight from Kirkwall or Stornoway to Glasgow and then go all the way to Sydney without ever having to go back through security. As a result it has to be as rigorous as Heathrow. I hope that makes sense?