I'll also jump on the GNER bandwagon here. It's one of the few really good train services in the UK. And yes, the First Class is excellent, whether in the InterCity 225 trains or the leased Eurostars. Many trains also have wireless internet access on board, which is a great way to pass the time for the slightly less interesting part of the journey (the first part).
To the en-route sights, I'd also add the view of the Tyne bridges as you sweep across the river just before entering Newcastle.
Also note that GNER have a 'Weekend First' scheme - if you're travelling at the weekend, there are very good value upgrades available - a certain number can be pre-booked, or you can pay (in designated carriages - not all 1st class carriages) on the train. Not all fares are eligible for the upgrades, but it's far from just being full fare that is.