I have been going on cruises with MSC for the past 3-4 years and there is no doubt that the number of brits are definitwly on the decrease and very much in the minority. Majority are usually Italian and german and sometimes Russian too. However they make a point (usually) of seating people of the same nationality together at dinner, so I'm sure you'll find some people to talk to.
I was on Xmas 2005 & there were a lot of brits onboard. Our BA flight from LHR was virtually all for MSC pax. Hope this helps.