Not in the same way that's true. Surely if all formalities are NOT complete before access is granted to the domestic departure area, there is nothing to prevent you passing illegal goods etc onto someone boarding ELSE a domestic flight.
1) Enter UK on arriving international flight
2) Clear customs and immigration
3) Gain access to domestic / common departure lounge
Where's the loophole that gives anyone access here without clearing UK Customs? Or are they treating T5 like Terminal 3 Flight Connections Intl to Intl without having to pay someone to do the passport checks?