Princess Cruises do a 7 night cruise on Sea Princess on 12th July calling at Amsterdam, Oslo, Copenhagen and Helsingborg. I did this cruise a couple of years ago and it was great. The crew are multi national but speak very good English, I didn't have any issues with them.
I've no experience of Costa but have sailes with MSC and did the baltic cruise last July. I would say that it is very true that there are not many british passengers. On the baltic the main group appeared to be german and russian (Probabaly because it was possible for both Nationalities to join bthe cruise from their own Country later in the week), the remainder were mainly italian. However i would not say that I've ever had any language problems. All the ships officers and entertainment crew are Italian but all speak English. Announcements and all written amterial are in 5 languages of which English is one. All the bar nd restaurant staff are from all over the world but understand English to varying degrees as do the cabin stewards. Spa staff are mainly Balines but whilst not having fantastic English skills certainly do the basics. I have come accross some fellow Brits who had some issues but in my personal opinion that was because they had particularly strong regional accents and made little or no effort to modify it or there use of local idioms etc. They will always seat you with other English speaking passengers at dinner and you will have an English speaking guide if you elect to go on their shore excursions. I quite like the European mix of cultures but it is not to everyone's taste. if you prefer to be surrounded by mainly British passengers, then I would not recommend that you book with MS.
Try Royal Caribbean and Thomson.