We did 10 nighs Ambassador last Sept round UK. I don't posses a DJ, suit, or even a sports jacket, and did not have an issue eating every night including 2 formal nights in the main Buckingham dining room.
Trousers, smart shirt with tie or dickie bow - the same as our last 15 Marella cruises, so don't worry.
Staff, entertainment (Peel), food and drink were all good, and UK passengers also as friendly as Marella.
The main issue we worried abut was fixed dining rather than freedom dining. We chose 1st sitting on a table for 6, and happily it worked out OK