Music and video sales don't attract taxes, but application sales do. I'm not sure exactly why that is. If they're accounting for gift card sales under whatever rule allows them to dodge taxes on music and video, they probably can't later apply the payment to an application. Be careful what you wish for; the "fix" might be to pay tax on everything.

As for availability, media distribution rights tend to be divided up by country, with one company holding the licence for Canada and another for the UK. So if one licensee doesn't want to offer the whole catalogue to Apple for some reason, you'll see a difference in availability. For some content, there may simply not be a distribution agreement for Canada.

Using a UK account in Canada is against Apple's rules, but I'm not aware of any IP address restriction.

Thanks for the clarification. It makes more sense now! Apple told me that trying to buy material with a UK credit card on a Canadian billing address would probably not succeed! Maybe the record shops aren't doomed yet!