It would certainly be strange to start a route that doesn't have too much potential for profit so far from their usual market, in a territory and with customers they don't really know. They do operate ships year round in the North Sea between the UK and the continent but I don't think the Atlantic can be compared to the North Sea.
The route also seems rather short for a cruise ferry service as you say, plus the fact that they don't have any ships available for it. That is of course unless they decide to close one of their North Sea routes in their upcoming business review, but the only route to be possibly closed is the Zeebrugge - Hull route which is served by two 25-year old ships, not really attractive ships to start a new route I think.
Kind regards
Cedric