Far be it from me to criticise check in staff (I used to be one) but they are not Border agents and nor should they be. If UK Govt wants to check those going out then they really should be doing the job, like they are for train and sea travel. Anyone wishing to slip out will do so by air unless a proper check is being done.
So a person slips out by booking a ticket from LHR to JNB, for example. They may have overstayed by 3 or 4 months, but should they check in online how is it going to stop them from boarding? What if they online check in a put a load of gumpf into the system? Even if they drop off a bag then the passport is highly unlikely to be scanned as there is no need, the system generated a boarding pass based on what was put in.
I can't think of a time anyone has checked to see if the APIS details I put in online match what is actually on my passport. I do wonder if this will now change. Anyone who has online checked in will either need to verify APIS before going through security, causing delays, or at the gate, by which time it'll be difficult to stop anyone...and also slow down boarding.
On the subject of verifying who is in cars etc. I've crossed Dover-Calais numerous times in the past few years and I think only twice (out of maybe 4 or 5 times) has anyone from France (based in Dover) actually checked my passport, on one occasion the booths were all closed. I got all the way from West London to the Slovenia/Croatia border once and my passport didn't leave my glove compartment until I got stopped there! Also, at said border, they took all the passports of the people in my car, gave them a glance, didn't even bother to look in the car, stamped them all and away I went, same a few days later when I was leaving Croatia. The Slovenes were a bit more official, being an External EU/Schengen border...however a few days later in Calais, no one from France was there to "check us out".
