In 2008, when it was worth doing (not now!) I did Glasgow to Penzance throughout (Cornish guard out of Glasgow, Scottish guard out of Plymouth!), then Penzance to Aberdeen on the Penzance to Dundee HST, followed by a EC HST from Dundee to Aberdeen.
We went in 1st on the way there, as it was a weekend and a Valenta was on the front. On the way back we went Standard but had some 'dining', which involved
Max bringing a pizza from Pizza Express on board at York.
In fact I did something like that twice, so may have the occasions mixed up slightly.
Scenery is best in Cornwall, along with most of the parts along the coast. The middle bit is not good for scenery.
No way would I do that on a Voyager, but you can get most, if not all, of the journey on HSTs anyway, if you time it right.
You could also combine it with buses from/to Lands End/John O'Groats for the 'ultimate' journey