It's not posh to say supper or any other varient, it is sodding regional, there is no status attached to what meals are called but on MN where they are all MC until an election comes around and everyone is all of a sudden WC.
It means nothing in a wider context and anyone trying to apply such 'non existent' class parameter onto such sayings are the complaining to the local paper in green ink brigade.
It's just sodding dialiect where no one is wrong and no one is right, the imaginary hierarchy hasn't existed since the late 1800, anyone wanting to prove a narrative are usually the less educated or social climbers who pretend they are working class come an election.