You can use a credit card in the online Apple Store in Canada.
I don't think that you can use a Canadina credit card in the UK store. Is that what you are trying to do?
On an iOS device using iTunes or App Stores, you have to use a form of payment native to the country of the store you are shopping in. So, if shopping in the UK store on your iPad, you would need a credit card issued by a UK source, with a billing address in the UK.
All the iTunes and App stores are country specific, and use may even be blocked if accessing the store from a foreign IP address. You cannot use a Canadian form of payment in any other iTunes store than the Canadian one.
You cannot gift items from the iTunes or App store to another country either - http://support.apple.com/kb/HT2736?viewlocale=en_US
Gifts are only redeemable in the country of origin, nothing new about that.
I don't know what the commonwealth has to do with it - Canada and the UK are independent nations. The commonwealth is largely a symbolic union - each member state has no legal obligations to any other and theres is nothing about the commonwealth that subverts independent laws about cross border commerce nor individual national copyright limitations and local taxation.