The angle taken by the Left with regards to the Geneva Conventions will be that the US and Britain are no longer covered by the GC since we are "torturing" detainees and invaded a sovereign nation illegally.
Therefore the Left is going to spin it that these 15 sailors can be tortured/humiliated as Iran pleases.
The Left can attempt all they want to say the US/UK are not covered by the Geneva Convention. But it will sound utterly retarded especially since the UK isn't doing anything on extraction of prisoner information.
If they wish to side with the mullahs torturing the UK soldiers, they will isolate themselves. But that's not new for those who spend their lifetime defending Stalin, the Soviet Union's various invasions, North Korea, Chavez, etc.