To someone not from the UK, generally the stereotypes that comes to mind when someone says 'British' are 'English' ones. It's the predominant culture associated with the UK amongst non-Brits for the most part. Say Scottish and they'll imagine kilts for example, but if you say Britain, the first thing to come to mind for a lot of non-Brits (and certainly Americans) is generally going to be London, Big Ben, etc. So since the series is about international stereotypes, even when calling him Britain, it kind of makes sense that they'd go for an English accent. And as you said above, England the character is definitely a stereotype of an Englishman, not a Scotsman or a Welshman.