The only word I'm familiar with which has initial /kn/ is the word 'knesset'. The earliest quotation attested in the OED dates back to 1949. So the word seems to have entered the English language long after the change from /kn/ to /n/ had taken place.
Correction: In the meantime another word with initial /kn/ popped up in my mind: 'Knossos'.
Being a modern loanword, however, does not prevent it from being adapted to the usual sound restrictions: