As stated, we just sailed Silhouette out of Southampton and a kettle was in the room other ports not typically. I will say many of the Brits we spoke with said they could not get a good cup of tea on board. . I can't confirm as I drink coffee and with the beverage package I order mostly lattes. We've never done P&O but I understand they obviously cater to a British crowd. On Celebrity I think you'll find it split between Amercian and British.
Most European cruises are port heavy and a lot of time spent off ship so beverage package upgrade can be challenging getting your money's worth and better to pay for overage.
Cocktail and or sommelier service in the dining room can be good or just meh. I found this last trip the sommelier service was very good and we had My Time Dining so we had a few different ones. Cocktail is probably the most challenging. We typically grab a bevergae at a bar prior and either get a drink to bring to the dining room or we are ready for wine with dinner anyway.