Many of the cruise lines are very well equipped for wheelchair access but have to be careful with some of the British smaller ships - the crew will always do the best taht they can and help.
All ships will have disabled facilities but it very much depends on the ship that will suite you. I have never come across a crew member unwilling to help anyone but some of the smaller ships maybe more difficult to get around on.
Most of the cruise lines welcome passengers who require the use of wheelchairs, and most of the ships have specifically adapted disabled cabins onboard. They do normally stipulate that you must be accompanied by an able-bodied companion. When making your booking just point out straight away that you will be travelling in a wheelchair, so the correct cabins can be checked for you and any other necessary arrangements can be discussed. Why now check out the 'Accessibility' section on the left hand side of the website for any further advice.