CBC's Dieppe from 1993 about the Canadians who helped the British Army during WW2 tries its best to convince as British. Set mostly in London, filmed entirely in Toronto, with a cast of Canadians and Canadian-based Brits e.g. John Neville, Nigel Bennett, the accents from Robert Joy and Victor Garber as Mountbatten are more accurate than an American production but while Garber is convincing, Joy is still quite over-mannered, but a lot of the accents are passable, especially from Dublin-born Gerald Parkes, Doc in the US/Canadian/Irish-shown Fraggle Rock. However, Gabrielle Rose, the Canadian-born actress known to us from her role in Rising Damp as Brenda during her post-drama school stay in Britain's accent as a British woman is nowhere near as convincing as her accent as Brenda (which although seemed to go from region to region, no more than any other British actress of the period). Certainly, there's more knowledge of British life and culture (pub songs, a scene set during the making of In Which We Serve), and Toronto is shot in such a way to mask the locations, lots of tight closeups, and a lot of Canadian-British actors in small parts. It almost works. But perhaps a few non-Canadian based actors may have helped.
Garber also played Prince Charles in the other godawful William and Kate film, with American Jane "the Betsy" Alexander and Jean "Joan in the US Reggie Perrin" Smart as Camilla. ITV comedy drama bit-part regular Mark Penfold plays Phil the Greek.