Check different companies websites. march and April are a popular time for repositioning cruises when ships are moved from the Carribbean to the Med or other parts of Europe for the summer. Not sure if you will get one from barbados though, most seem to be based at Miami or Fort lauderdale. They might allow you to join the crossing a few days later if barbados is one of the ports of cal.
You could be lucky. P & O Cruises have a ship, The Arcadia, leaving Barbados on March 21st sailing around some of the Islands before setting off for Southampton to arrive UK 4th April. Cruise Number is J806. Total days 14. Happy hunting and see you on board if you choose it.