We can negotiate with the host nation to return them. For example Pakistan rely heavily on trade with the UK. We can cut that overnight until they play ball. The same with military support. Failing that we can work with a third party nation.
If they've committed a serious crime such as rape or murder they can face the death penalty, which assuming we're in a position to remove immigrants, we'll be in a position to implement capital punishment.
You're making out like the removal of an immigrant is a difficult task. It's merely a matter of logistics.
I put this to someone else already but:
For the even small number of people who are unwanted in Britain but also unwanted in any other country and nobody will take them off your hands, what do you propose to do with them?