Is the purpose to deter would-be illegal immigrants or to apprehend those that are already in the country?
I cannot see it acting as a deterrent, there are already deterrents to becoming an illegal immigrant to the UK and yet they are still coming.
It is true that illegal immigrants probably will not have sufficient resources to obtain false documents in the same way that I outlined above for terrorists. This will mean that they will tend to avoid the places and situations where identity may need to be determined and an ID card be presented.
The police will not have the power to demand ID cards and therefore can only try to establish the identity and immigration status of those suspected illegal immigrants that have been arrested for some offence.
Immigrants try to be inconspicuous and avoid the attention of police, although if they are turning to crime due to the deterrent to illegal working they will increase the risk of arrest.
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