Both the sleeper trains to Scotland (a number of destinations) and Cornwall (ditto) are great fun (but I love sleeper trains - they can be quite romantic in a cramped bang your head on the ceiling kind of way...


- for those who like a lot of laughter in their relationships anyway!!). It's a nice way to go to the Eden project or anywhere else in that area.
There's the overnight boat to Santander from Plymouth - but that is a bit of a faff - would be worth it once you get there - I would highly recommend nearby Bilbao - one of the best "city breaks" I've done. The train and boat to Ireland is nice and the section that goes from Chester to Holyhead is really really really lovely - mostly along the coast on a tiny tiny train.
If you're willing to take Eurostar there are any number of night trains and high speed trains to various destinations in Europe - for example, I was in Alsace at the beginning of June on the "Route de Vin" near Colmar (managed to get a week long INSET there...

) which as the name suggests is a fabulous place and looks like a giant set for Hansel and Gretel. I would also recommend Brittany - Mont St Michel, gallettes, more wine (can you see a theme?).
(I don't know if you are at all interested in cycling - but taking a bicycle, especially if it's a folder, to any of these places opens up lots of options - we've done some great cycling and staying in Gites in northern France, Pas de Calais, which basically involved jumping on the train from Herne Hill down to Dover, hopping on a ferry and then cycling from Calais to some wonderful little places with stunning scenery).