Fred. Olsen Cruise Lines has a very high ratio of single cabins (inside and outside) and cater well for the single traveller. Better than paying a single supplement.
They are smaller more intimate ships, which make it far easier to make friends than the larger ship.
Island Escape do single cabin.
Some P&O Cruises are fairly low single supplement. Best bet is to phone up one of the Cruise Specialists and talk to a Cruise Consultant, they are the experts and should be able to help you find a good value cruise.