Distances and travel times are very short in Benelux, so your route really doesn’t make much difference — backtracking is hardly an issue. You might save some money by taking the bus, but the train is so much faster and so much more convenient for this area that I’d stick with the train.

Possibilities: Eurostar to Brussels, then Luxembourg, Bruges, Amsterdam and Thalys to Paris, Eurostar back to UK.

Fly or Eurostar to Amsterdam, then Brussels, Bruge, Luxembourg, Thalys to Paris, Eurostar back to UK.

Remember that the high speed trains must be booked well in advance if you want to make use of the cheap rates. Local trains & intercities don’t require reservations.

BTW: I would forget about Luxemburg, there are many much more interesting places to visit. E.g. Düsseldorf, Antwerp, and numerous old towns in Holland/ Belgium.