As far as calling charges are concerned it is not your business where someone is located. If the number you will be calling is a US number you will pay charges that apply for calls from the UK to the US.
The person receiving the call will pay roaming charges if they are traveling in another country. I don’t know what these charges are for Americans traveling to Canada, it depends on the wireless provider, their roaming plan and so on.
They will pay international charges imposed by their US wireless provider (these charges are agreed upon between providers from US and Canada in this case). Basically the charges are higher than they would be when calling from the US because there is also the roaming charge. As the receiver of the call you will not incur any charges.