Tipping is a matter of culture.
In some countries (eg Japan ) any tipping is not needed at all ,
In some other countries (i wont shame) need to tip to get a table of any sort even if you have a reservation.
In UK tipping in advance, is not customary, and frowned upon, not the thing to do. I would never do it, if that's what it takes avoid the place.
Cunard is supposed to be British. If tipping in advance, or tipping to get a better table, is starting to seen as acceptable on Cunard , then this is another indication of creeping Americanisation. However never seen it on Cunard, it's certainly not to be encouraged.