Can I get a UK passport if I was born in Canada in 1978?
No; Canadian and British nationality were separated in 1947 by the Canadian Citizenship Act, which went into force in January of that year. Before that, Canadian citizens were British nationals who happened to be living in Canada; after that, Canadian nationality was recognised, with Royal Assent, as separate from British. Canadian independence didnt happen in one fell swoop in 1867 with confederation or in 1982 with the charter, but rather in dribs and drabs over the course of a century. The right to issue its own passports happened to come in 1947, well before your birth.