So they just immigrate illegally?
Europeans which now includes most of eastern Europe can come and go as they like. Even if its a convicted murderer there's no information sharing or at least a suitable one.
Non-Europeans usually make their way from Afghanistan, middle east, Africa...etc to Europe, head to France and camp in Calais waiting for an opportunity to cross over to the UK in boats, usually cargo, lorries...even the Euro star has been known to carry some clinging on to parts of the train.