It is possible to get a cabin with no single supplement, but you have to check teletext and occassionally a good deal for singles will come up. I managed 3 last year with no single supplement and 1 this year with only £100 extra. Sailing again on 28.09.07 with no supplement but haven't cruised with this line before so don't know what it will be like. Cruise lines I have already been with are Fred Olsen, Orient Lines, Classic International, and the one I am about to cruise with is Monarch Classic Cruises, the last two can be contacted at www.cic.co.uk, take a look at their brochure which you can download.
Most of the American ships ask 100% which is just outrageous. Shop around and if you can be flexible with dates and itineries you should get something. Good luc.
Try QE2 who still have cabins designed to accomodate single cruisers, also they have programmes designed to help singles meet and mingle.
I took a 8 day cruise on the Van Gogh from Falmouth Oct 2006 arranged by Travelscope, which not only was there no single suppliment a £100 free bar tab per cabin was included. Very good value. The only grip came from passengers who used the free coach especially from the north of England.