What is the problem here, for once it is saving the country money. An even better idea is for the illegal immigrants to pay the £1500 to the government for them to transport them out of the UK and then we would know they have gone.
The only point is, they are illegal, so we do not know they are here and even if we did, they only have to get rid of their passports and they will most likely be allowed to stay. If they have their passport, they can be deported, because we know their country of origin.
Shall we all try this when we go abroad and lose our passports, but then again other countries do not adopt our ridiculas rulings.