I notice you are saying All Cruises. As a person who has cruised as a single every time so far, my advice is to shop around, don't just go for a single cabin, it is quite often the case that, single occupany in a twin cabin, depending on the cruise and the time of year can be cheaper than a single cabin. Your best bet is to contact one of the cruise agencies, why not try the freephone number in the header, and ask their advice, but have some idea of where you want to go and the time of year, it will help.
Which cruise company, which ship and which cruise?? Some ships have single cabins, others you have to reserve a double cabin for single occupancy.
How long is a piece of string? When do you want to go? Where do you want to go? Who do you want to sail with? Are you bothered about any of these questions? You could pay £300+ a day for a single cabin on the QE 2 at the moment, OR, if you are not so fussy, there are a number of cabins available for single occupancy being sold for between £75 and £100 currently for cruises during the next few months across a range of cruise lines and destinations! A call to Fred Olsen could be especially productive at the moment. Just don't forget that fuel surcharge! Happy cruising.