The difficulty that the Prime Minister has in refusing to provide more funds to tackle the problems in Calais is that, by agreement with the French, the UK border is actually situated in Calais. UK Border Agency staff are situated there and “illegals” who are intercepted there do not have the opportunity to set foot on UK soil.
France is continually threatening to rescind that agreement. If the border was moved to Dover all those currently discovered hiding in lorries who are ejected before crossing the Channel would actually make it to the UK (as the French, as we know, are none too keen to be over-zealous). Of course the remedy for this would be for the UK to return the miscreants directly to France, as is allowed under the rules. But the EU has deliberately blurred the lines between what should happen under UN conventions and what does happen under EU protocol. The end result, as we have seen of those who do make it, is they are allowed to remain here.
The root cause of this reverts to the freedom of movement. Once they have gained a foothold in mainland Europe the migrants are free to roam wherever they wish across the Schengen nations (and would be free to travel to the UK had we become a signatory). If they had to be contained in, say, Italy, the mayor of Lampedusa may not be so keen to announce how proud he is that his island was the “destination of choice” (yeh, right!) for those travelling from North Africa. If they had to remain within Italy he may find his pride is somewhat tempered by a population that would quickly become cheesed off with having to deal with the numbers involved.