There are still sample menus on the website but for some reason they only show a couple of choices.
https://www.cunard.com/en-gb/activity-types/dining/britannia-restaurant
We have links to actual menus posted by @rakkor in the Welcome sticky.
It's always useful to see actual menus, it's so disappointing to go to a restaurant and find out the menu you are given is completely different to the one you've been looking forward to.