I did preface with '....depends what segment you're on...'. According to the calculation for FCC, 75% of the Singapore - Soton segment is affected so the offer of full refund seems entirely reasonable. Hong Kong - Soton a bit more moot but I would think there is still a good case. The Australia - Soton segment or the full World Cruise itself must lend themselves to some interesting refund/compensation discussions.
I'm loath to feel too sorry for Cunard as a) they have probably got some sort of insurance to cover this - sure their premiums will sky rocket going forward but, for now, I'd be surprised if they're not covered, and, b) some of the fortunes they must have earned on drinks in the past will soften their blow.