LHR tries to do 100% security scanning of pax between flights
This is a mandatory requirement -
all pax and baggage transferring at UK airports mustbe screened - it must not be assumed that an arriving flight is "clean".
UK/Ireland Domestic to UK/Ireland Domestic should also be screened, but pax who know where they are heading may circumvent this by remaining in the Domestic gate area instead of following the transfer route.