Do you have a few months to spare? If so World Cruises from Southampton might be something that may interest you. These cruises are often 90 night plus cruises; some are even over 100 nights in length.
World cruises tend to visit most regions, but not every one. They do not tend to visit the Norwegian Fjords or the Baltics; more often than not they are focused on the Middle East, Caribbean, Australia, South America and the States though not in that order.
Below is a list of World Cruises from Southampton; these are in alphabetical order rather than date order. There tends to be only a handful of world cruises each year and so this page will not show many cruises. I have not included segments of world cruises as these require a flight home; this may change in the future.