If Iran or NK nuked the UK, I should think it a relatively simple task to glass over their major cities. Annilihation of NK would likely result, and severe damage to Iran. I would hope enough to dissuade them from doing it again, so perhaps a secondary deterrent if the primary failed. I.e. the threat of H-bombs was not enough, so would the actual use of a 250kt H-bomb on downtown Hanoi now do the job?
I'm sure most of the neighbouring states may have something nasty to say and perhaps write letters to the Telegraph, but that's about all.
China on the other hand is a whole different kettle of rice. Put India in that bracket too, if push came to shove.