1. Having a border agency with a strict remit is absolutely vital. Britain is far too crowded as it is, and some of those wishing to enter are not benevolent to our interests at all. A terrorist attack on Britain would be more highly prized by our enemies than one on say, Holland or Denmark.
2. Whilst it is relatively straightforward to check a single manifest against 200 - 300 individuals, that is not the way airports work. Arriving passengers are mixed together before arriving at the border control, so instead of 200 or so, there are suddenly several thousand, from different continents and displaying different levels of risk.
3. Governments are completely useless at doing anything. They use expensively-employed staff who are generally under-utilised. They concentrate on political correctness rather than getting the job done. They employ the worst managers (the best wouldn't dream of working for the public sector) and weave themselves into a web of bureaucracy.
So what is the answer?
I would suggest privatization of UKBA but with third-party independent auditing. A Service Level Agreement to control delays and thoroughness of checks. Personal legal liability in criminal law of employees and Directors who fail to carry out their functions correctly.
There may be a need to redesign airports to separate passengers from different origins, and to process arrivals on a flight-by-flight basis.