crapatcryptics
Quite obviously the lorry was not inspected at the Port where it entered the UK. It is not a matter of them being bad at their job, it is a matter of there not being enough personnel to do the job. It would appear that not all lorries are inspected, but a random selection.
From June 2015:
// Mr Vine predicted similar incidents would only become more common as the Border Force found itself increasingly unable to cope with the influx of migrants being smuggled across from the Continent by human traffickers.
More than 40,000 suspected illegal migrants were stopped at UK ports and border crossings in the year to April.
Mr Vine said it was not enough to rely on spot checks of the hundreds of vehicles passing through Britain’s ports each day, but that at the same the Border Force was losing experienced staff able to spot those lorries which might contain illegal migrants. //
If we want to stop these incursions we will have to put sufficient manpower on the job, but the Government are not prepaed to do that.