I'd guess that many of the pax travelling before or after a cruise have plenty of time - the important thing is being (almost) 100% certain of travel and not being cancelled for any reason. After a werk's cruise, people are not in any great hurry. Furthermore, on days when there are no cruises arriving or leaving Southampton, I'm guessing demand will be low or uni-directional. Some days feast, other days famine.
I'm not convinced that a 2x daily high-cost-per-passenger E145 is really what a SOU-MAN route needs. On some days that will be nowhere near enough capacity, while on other days it will be far too much. I'm thinking something closer to a 10+ year old A320 which can be flown just on the specific days to tie in with cruises.