Why is that strange? I'm an American and I like both those things. It's stereotyping to assume that people from a given country are only allowed to like things from that country. Besides, England and France are next-door neighbors, 24th-century Europe is a more integrated society than it is today, and they have transporters that would make it effortless for Picard to have commuted to school in Nairobi or Seoul if he wanted, let alone London. The idea that there still has to be some impermeable barrier between Englishness and Frenchness is a failure of futurism.
I'd assume it's the same one Goodman used in Federation: The First 150 Years. In developing Rise of the Federation, I thought about trying to "explain" the Denobulans' apparent rarity in the future, but it seemed unnecessary when I realized that ENT had already established them as a gregarious people who like to live close together in dense communities and share a single continent. So it followed that they would be stay-at-homes by natural inclination and only a small percentage would ever want to leave Denobula.
In terms of the tea reference, there's actually a misprint in the book that ended up accidentally cutting a footnote explaining why everyone in France has an English accent, and the earl grey footnote was a follow-up. Frustrating. In terms of getting rid of the Denobulans, I actually go further in this book, and it was used to make a couple of other statements about whether everybody in the Federation is truly oriented towards peace.