It depends on the aircraft's final requested cruising level and the performance expected in the climb.
Aircraft with a RFL of 240 and below (230 in reality, as it's deemed an eastbound route heading south), will use the MONTY SID The NOKIN one requires flights to be FL250 or above in the vicinity of ELGAR (I think?) where it becomes an upper air route somewhere over mid Wales. Also slower climbing traffic that would be unable to remain above the various bases of controlled airspace on N862 would be expected to file a MONTY SID.
I hope this helps?