I've been reading the various reports on the "easing" of immigration controls, and my instinct is to suggest that the critics are aiming at the wrong target.
When I have returned home through Heathrow or Gatwick during the past five years or so, I cannot recall one occasion when my passport was inspected by someone whom I would describe of ethnic British descent. I recall the first occasion very clearly, many years ago, when it was examined by a woman wearing sari, who in spite of my British Passport asked me if I was normally resident in the UK. Frankly, I was quite shocked at the time.
Now I don't doubt the loyalty of 99% or more of our immigration and passport control officers, but it only needs one individual whose loyalties lie elsewhere, to let in an unknown number of unwanted undesirables. At Heathrow this would be particularly easy, an officer could advise an immigrant when he was on duty, it would be quite easy for someone to hang around in the transit lounge until the appropriate time, and then choose the queue where the contact is inspecting passports.
We have already had cases of visas being wrongfully issued, I believe someone of Nigerian descent was involved in a recent case, and before that, an employee at our Embassy in Delhi.
Some countries (amongst them Switzerland, so I am informed) require all border control personnel to have been born in the country of parents who were also born there.
Some unwanted immigrants may have got into the country whilst controls were relaxed (but they would have needed to know that this was happening), but I am far more concerned about those being let in by corrupt or disloyal officers in the Border Control Agency and other immigration services.