* People from Great Britain are British.
* The most common languages spoken in Great Britain are English and Welsh.
* Canada is a bilingual country, with English and French as its official languages.
So, to answer your question more directly:
* British people living in Canada would speak English (and possibly French, if they are bilingual).
* There is no unique language specific to British people in Canada.
It's important to remember that language is a complex topic, and there are many factors that contribute to language use. For example, a British person living in a predominantly French-speaking region of Canada might learn French to communicate with their neighbours and community.