Presumably they’ve ruled out prisons in northern European countries perceived to be too humane; imagine the Sun/Daily Mail thundering about “hardworking taxpayers’ money spent to give criminal scum holidays in luxury Finnish prisons”, along with a photo of a cell that looks vastly better than the typical London rental opportunity. So I’m guessing they’ll be asking around, say, Turkey, Morocco and various former Soviet republics. Possibly the US as well, though that may involve leaving the ECHR.