I watched a show following police enforcing immigration laws in the UK. They would find someone who was an illegal immigrant. Question them. Try to find their passport. Fail to do so and then, being unable to deport a person without a passport, release the person in question and ask them to report to a home office site once every two weeks. Unsurprisingly, most didn’t. Near the end of the episode one of the officers talked about how he felt when his team of 4 had found a single illegal immigrant, spent 8 hours with them trying to find out where their passport was and eventually had to let them go.
It’s sad that immigration enforcement is so poor. It seems to be mostly down to how bad the laws are. It’s sad generally how inefficient the british legal system is.
I recommend The Secret Barrister for more on british legal disfunction.