Hi attard and welcome to Cruise Chat.
Sorry to say this, but I think you're cutting it way too close, particularly with an international flight that you did not book through the cruise line. You won't even get out of Gatwick until 12:30, between going through Passport Control, claiming your luggage, etc. If there is any kind of delay with your flight, you're in trouble.
You said you purchased NCL's transfers. Are you within the limits of the last arrival time?
My first choice for a 2pm deadline would have been to arrive the day before and spend the night in Dover. If that's impossible, I suggest you get travel insurance that includes provisions for missing the ship. This will help pay to get you to the first port of call to meet the ship. Is the first port stop the day after you sail, or is there a day at sea in there first?