I'm going there in August-September so I can give you some advice but I haven't been there (on trains at least).

- Inverness - Kyle of Lochalsh

- Mallaig - Glasgow : Glenfinnan Bridge (Harry Potter), Corrour in the middle of nowhere,...

Those railways are incredible from what I've seen or heard. We're combining the two with 1 night on the Isle of Skye, you need to check bus times to Armadale (and then ferry to Mallaig) because they only run 2-3x daily (weekdays). If you want advice send me a private message

- Carlisle - Settle (- Leeds) is pretty scenic too. We're going to do Carlisle - Leeds - Manchester on one day, Liverpool is an extra 35-40 min so it's fine too.

And I'm sure other people will make suggestions

I was there just a couple of weeks ago, London-Edinburgh-Glasgow-Fort William - Glenfinnan and back to Glasgow. (Glenfinnan just to turn and see the viaduct). 

I don’t know how it compares to the other routes, but the route from Glasgow (especially after Crianlairch) to Glenfinnan (and probably on to Mallaig, but I didn’t go there) is exceptionally scenic and beautiful. Mountains, lakes, sheep, stunning spots…  So I can definitely recommend that, but I imagine many of the other routes up there are also very beautiful. 

In Glenfinnan, you can walk to the viaduct and (if you are there in time for that) see the “Harry Potter train” cross it. A very nice walk, around 30-45 min there and a similar walk back. 

The line Perth-Inverness is also quite scenic, maybe a bit less dramatic landscapes than, those in Western Scotland  but it gives you a very good Highland feel.

I also liked the wide views on the mouth of the Tay (I travelled from Perth via Newburgh, maybe also good views over Dundee or Leuchars) and the Firth of Forth (Over Burntisland, Kircaldy). Then you’ll also get a ride over the impressive Forth bridge :)

Going to Scotland, I prefer the West Coast mainline, over the East Coast, when it comes to scenery. Avanti has also better seats than LNER in 2nd class. But sometimes very restrictive view from the windows.

When it comes to Scotland thibcabe has already said it all (and I was there in April 2023 and can only confirm). In addition to the scenary outside, I found it a particular atmosphere as we all boarded the training knowing that we would be in it for over 4 hours but with very few places to get off along the trip. Like going to the moon together. 

But then you also mentioned Wales. Machynlleth to Pwllheli is definitely scenic too and has the additional feature of passing right along the shore. 
This is also a feature of the Exeter to Paignton line in the very South. When the sun was shining it was like being in the Mediterranean!

May I add the Newcastle to Edinburgh rail section. The rail line passes very close to the north sea near Berwick. The waves crashing on to the rocks is a site to see. This would be part of the London to Edinburgh line if your going that way in any case. 

Hoi @OuwensG 

Ik zie in je profiel ‘Nederland’, dus dat maakt het wat makkelijker om te reageren. Wij zijn vorig jaar met de trein naar Engeland en Schotland geweest. 

Onze reis: London - Edinburgh - Inverness - Isle of Skye (via Kyle of Lochalsh > bus naar Portree), Glasgow (Portree > Armadale met de bus, ferry naar Mallaig, trein naar Arisaig voor 1 nachtje, trein naar Glasgow), Manchester, Oxford, London. 
Overal zijn we een paar dagen gebleven. In totaal ca 3 weken weggeweest.

Kijk ook eens op https://www.scotrail.co.uk/scotland-by-rail/great-scenic-rail-journey
Daar hebben ze de Schotse routes mooi in beeld gebracht. ScotRail vertelt je zelfs wanneer je uit het raam moet kijken. ;-)

Alles op de reis was de moeite waard.

Als je naar Isle of Skye wil (zeker een aanrader!!!) moet je even goed puzzelen hoe je er komt met de bus en met de ferry. Bussen rijden bv niet alle dagen in het weekend en de ferry vaart ook op specifieke tijden. En die ferry tijden sluiten dan weer niet aan op de trein die vanuit Mallaig vertrekt. De toeristische Harry Potter trein rijdt nl over hetzelfde spoor en die ‘blokkeert’ de route. Wij moesten 3 uur in Mallaig overbruggen bijvoorbeeld. Het lukte ons alleen om het rondje tegen de klok in te doen en niet met de klok mee wat ons oorspronkelijke plan was. 

Om te kijken welke trein en route we zouden nemen gebruikten we steeds  https://www.nationalrail.co.uk 

Voor gratis treinreserveringen (voor alle operators) gebruikten we https://www.gwr.com. Gewoon een account aanmaken en losse reserveringen boeken. Heel handig, want dan heb je altijd een plek. 

Ik hoor het wel als je nog meer info nodig hebt. Een mooie reis gewenst!