Being that as it may, on an Alaska cruise somehow I managed to not pack my husbands formal pants. The shop was able to find a pair that fitted him well, and matched his tuxedo jacket, so the rental place may be an alternative at a pinch if M&S doesn't work out.
We turned out to be quite lucky as it happens, BA managed to forget to load over 100 suitcases of the guests on this voyage, so some set sail for 10 days on board dressed only in their pyjamas. The rental shop was very soon out of formal wear to rent. I didn't feel so bad having only forgotten 1 pair of trousers, not 100 suitcases. I also bet that those clothed only in pyjamas will choose a better clothing option for their next flights.