It appears Nuk3town bonus map was a GAME retailer exclusive in the UK (and for digital preorders)

Many players in the UK are not able to play the Nuk3town bonus map after getting Black Ops 3 from a retailer that wasn’t GAME UK. When Activision first announced the map, it was stated that “all preorders” could get the map, but as of this morning, Activision Support updated their document to state that its at participating retailers only. And it looks like GAME was one of the only ones in the UK (aside from digital orders).

EuroGamer was able to capture a screenshot of the support site:

Players that got the game from other retailers in UK are left now without access to the Nuk3town map. Some customers state that their orders from Tesco had a code, while we cannot confirm wider availability of that.

SOURCE: EuroGamer