IMO, it is best to do like the US and most other countries do. Which is to make them serve their time in the country where he committed the crime. Im pretty sure UK has a similar approach to make the convicted person do their time first there.
Then immediately after serving his full sentence (hope he gets a life term here), then deport them back to their country of origin if they are not a citizen.
It really is the best way because it ensures a person follows the conviction and penalties rules in the place where the crime was committed. If they were deported first, then they lose control over sentencing rules and their own country may not punish the person as severely as they should have been.
I understand the sentiment but the deportation can be after full sentence is served, and in this case, I hope it is a long enough sentence that he may not even be alive to go anywhere when done with sentence.