"Then he gave the punch line. They will not be voting according to what is best for the UK. They will back whichever party offers "the best concessions for the benefit of Scotland and Wales"

The SNP's raison d'etre is to take Scotland out of the UK. They only stand in seats in Scotland and their constituents (whatever nationality they are) all live in Scotland. It seems pretty obvious that if a cetain issue looks like it is to the detriment of their constituents then they will work towards trying to protect their constituent's interest. The SNP government in Scotland have no majority hence they have policies that they are unable to forward (ie the referendum on independence)and they have other policies which they can only proceed with by taking note of the wishes of other parties and trading off against that. It is obvious that at Westminster they will try to get concessions from whoever is the government.