All of those places have good links to the main line between Vienna & Zurich, from where you can take your pick of routes towards London.

Either up through Germany to Brussels, or via Geneva and then French TGV to Paris. Luxembourg is a but more off the main route, but has good links from Germany (Koblenz) and France (Strasbourg > Metz), then onwards to Brussels.

The Oetz Valley in particular is gorgeous, and has good & frequent bus connection from right outside the station.

Well, the routes over Germany will be a bit more scenic I think.

Some suggestions, but absolutely not the most straightforward lines:

You can also stop somewhere alongside the Black Forest (I liked Freiburg am Breisgau) and you could take local lines through the Black Forest, e.g. Freiburg-Freudenstadt-Stuttgart (with the quirky tram-train of Stutgart, if you like these train nerdy things ) or Freiburg-Donaueschingen.

Plenty of nice places along the road.

The Rhine route between Mainz and Koblenz is also rather scenic, best sit on the right in the direction of the traffic for the best views, when coming from the south.

In Koblenz you have a rather frequent RE to Luxembourg. Trier, which you will pass, also seems interesting, with lots of Roman stuff. But haven’t been there yet, although it’s on my list.

The line between Liège (BE) and Luxembourg, via Gouvy is also quite nice, passing through the Ardennes. From Liège you have fast and Frequent trains to Brussels.

But the line Luxemburg-Brussels, via Arlon is also fairly nice. But a bit of a step down compared to the route across North Luxemburg and Liège.

Many thanks to both of you.