If someone commits a crime in a country they are not a citizen of should they be tried in that country or sent back to their own country to be tried?

I recently saw a programme in which a British man was tried and found guilty in (I believe) Thailand for smuggling a fairly small amount of drugs. He got 20 years in a Thai prison and the programme was documenting some of his time there.

If he had been tried in Britain the chances of him getting such a steep punishment would be extremely slim, a couple of years in prison at the most I would say at a guess and his quality of life in a British prison would be far better than in the filthy Thai prison that he is currently residing in.

I’m all for punishing criminals and, unlike a lot of people, I am not completely against prisons (although I firmly believe they should be used as rehabilitation and not just as punishment) but where do we draw the line and who should decide what punishment fits the crime?