'Christian' names went out with the Cultural Diversity strategy, a number of years ago. Probably early 2000's . Replaced by 'First Name'. No doubt because there aren't many who would want to be recognised as Christians, in this country now.
As for the 'Surname' , it could be an extension of the same principles, and could be the reason why so many Baby
'Mohammed's were born last year. Mohammed's have another name also but write their first names as Mohammed.... which in our culture would be our second or Surname.
If that makes sense, and please note 'could be', as it's just a suggestion.
It would be wise to call them and ask, as I imagine you don't want to change your identity.