Well at least this explains why the UK Border Agency is so bloody useless

It's simply that they cannot afford to send all of the illegal immigrants, overstayers and foreign criminals back because the cost of the privately chartered flights is so expensive!
The theory, as far as it goes, is not bad. Apparently the Agency books charter flights to take bulk numbers of returnees back to their countries of origin. But then they fail to fill all, or even nearly all, of the available seats. It's like the opposite of Easyjet.
According to information released under a Freedom of Information request UKBA has spent £133 million in the last 5 years, which equates to an average or £5000 per person returned.
Eh? £5000! You could very nearly buy them a seat on a Richard Branson flight into space for that. £500 would be more reasonable and even that is a lot for a flight to Eastern Europe which is where most of them go nowadays.
This is what a spokesman for the UK Border Agency says:

It is right that those with no right to be here should go home and these flights still represent a cost effective way of removing in volume.

Yes I daresay it is a cost effective method but the trouble is that you're not removing in bulk are you?
Also:

Some flights are left deliberately unfilled because the UK has arrangements with host countries not to send too many people back in one go.

Why do we have arrangements with the host countries at all? What are they doing for us to deserve any kind of arrangement?
I would load them all up and simply open the door on arrival in Romania, or wherever, and shove them out.
Better still, I would compulsorily load them onto their national airlines without paying for tickets and let them reclaim the cost from their governments.
It is well beyond time that the increasingly feckless UK Border Agency was sorted out.