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!
It is not the current that is different between the UK and North America, it is the voltage rating and frequency as expressed in Hertz or Hz. Many devices can handle a range of voltages from 110 volt through to 250 volt and also varying frequencies. However, you need to look at the supply label on each device you are wanting to use in North America to determine if it is suitable. You cannot change a 110 volt, (standard voltage in North America), to 220 volt which most all devices in the UK are supplied. You can however adapt to 220 volt and use a grounded circuit to convert to 110 volt in the UK if coming from North America and wanting to supply North American devices. You simply need an adaptor.