I have to admit I don’t even think about train travel in NZ. It feels pretty nonexistent whereas in the UK it’s a reasonable option as they actually have a network.
I’ve seen variable train prices and generally compare to flights. Sometimes it’s more expensive than flying. Possibly even rent a car and drive although there’d be one way fees so maybe not.
You could have taken the train to Manchester then flown to any of the London airports then taken the various transport options into the city. Some of the ticket prices aren’t too bad although Manchester airport appears to have a terrible reputation.
I think I paid less than you to get from Euston to Glasgow. That might even include the Uber ride in!