Originally Posted by
ajax
All Eurostar passengers entering the UK clear UK immigration checks at the station from which they depart (e.g., Gare du Nord or Brussels Midi). There are no immigration checks at St Pancras. There might be customs checks (although I personally have never seen any) but I believe there shouldn't be as all passengers are entering from another EU country and thus, technically, a common customs zone.
There
are UK immigration checks at St Pancras, the reason being that passengers can board the Paris-London train at Calais-Fréthun where there is no UKBA employee as the passenger numbers there are negligible. Heaven forfend that any no-EU passenger might slip through the net.