Who in their right mind would go on a cruise without being vaccinated after what happened at the start of 2020. The bigger issue is how many countries will accept any sort of British holidaymaker until their own vaccination programmes protects enough locals and whether travel corridors are reopened which seems unlikely anytime soon. If they are then proof of vaccination is bound to be a requirement. It might be the case that ships sail without calling in at any ports.
At the moment the EU nations are lagging big time buried in bureaucracy and inefficiencies while just for once the U.K is well ahead of the curve.