Lance is it a non British airline that you are buying the tickets from, and/or are you trying to buy your tickets online from here in turkey? These are possible reasons why the payment is not going through, as they are the sort of fraud risk etc. that your bank and visa will have put a fraud prevention stop on. You probably need to speak to Nationwide.
And just a top tip do not try to make the payment again or force it through, as this may cause Visa or Nationwide to put a stop on your card, and then you will probably need to go through a whole lot of nonsense to get a new card.