I have a power adapter that works for Great Britain. Can I use the same adapter in Canada and/or the USA too?
No, you can't. They're different types of adapters.
What I do when I need confirmation is to check this website:
It has an EXHAUSTIVE list of each and every type, the voltage and the countries that use them. It's been handy for some of the stranger countries I've been to and hasn't failed yet.
Myself, I have a multi-adapter. The only country it doesn't seem to handle is South Africa, which has 3 giant prongs - bigger than the UK ones!
No, they are different types of plugs. People in the UK need to buy an adapter to get US or other plugs to work in the UK.