Posted April 9th, 2014, 09:35 AM
as you are based in the UK you could also look at the European based lines like CroisiEurope, Arosa, Transocean and Riviera. It has become customary to offer single-friendly cruises off season. Those lines also have lots of cruises shorter than 8 nights. So many options. Through your local TA you could also ask for the brochures of the UK land tours operators, some nice itineraries availabe there.
"Best" is always subjective...
Perhaps it might be best to start with area, itinerary and time span as well as season preferences and take it from there.
Transocean Belvedere Danube April 2013
English Channel Crossings 1984 to present:
Calais - Dover
Ostend - Dover
Dutch island hopping 1980's
Many Rhine river short trips