Are you absolutely sure of that? I very much doubt it, because it would be too easy for someone to enter the UK sneakily if they were.
Indeed I am 100% sure, and others confirm below. The boarding pass check that passengers starting their journeys at Heathrow T5 go through before security is essentially one way, so transfer passengers can't go back through it. After this check, originating and transfer passengers are mixed.
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They typically were until about a year ago; there's now separate security zones (though I think still shared with priority security?). Security guards are there to stop anyone going land-side.
Strictly speaking, the southern security checkpoint was essentially unused until about a year ago, when they started routing transferring passengers through it. (It used to have one queue open for priority security and that was it, IIRC.) I presume this was always planned for capacity expansion?
Not sure this matches my experience. There were not separate security zones when I passed through recently as a transfer passenger. Maybe it depends how busy it is? And as far as I know it's always been possible to use both the northern and southern security check points (as an originating passenger) since T5 opened in 2007.
