AOG
Cards themselves can be forged, but we also have to consider the IT infrastructure which would bridge both the NHS systems and those used by the Immigration services. This is how forgeries would be trapped.
First, everyone in the country would have to be registered, and then the two systems would have to be 'bridged'. I work on systems like this for a private company, and without boring you with the details, it's a nightmare to implement successfully, because you have to take account of different database back-end solutions. Data from the two systems would have to be delivered, anonomized, secured, backed up, updated...the list goes on and on.
That is all prior to the system being built if course.
Here's what happened with a £3.8billion system implemented for the Department of Work and Pensions a few years ago:
http://www.computerworld.com/s/article/97853/U.K._government_hit_with_another_large_computer_failure
Can you imagine the furore if a hastily cobbled-together system like the one suggested for NHS ID cards were to suffer a similar failure?
Finally...should we not be listening to experts in the Health industry, both providers and managers, when it comes to resolving this issue...rather than MPs, whose primary objective is to staunch the flow of supporters leaving for UKIP?