Unless cost or time are significant factors in the decision, I'd always choose first class travel on LNER or Avanti over flying. (My own preference is for Avanti, but there isn't much in it.) The train is more comfortable, the station experience better than the airport, and the hours fly by when you're being served food and drink in a comfortable seat with lots of legroom and fine views through the window. Both train companies have free WiFi, although it is patchy (as is 4G/5G) in the more remote parts of northern England and southern Scotland.

If there are more than one of you travelling together, or if you are over 60, you might save money by buying a railcard that gives you 30% off rail fares, even if it's only used once.