I haven't had any problems buying books from Kobo - though I have noticed that when I download my purchases, they show up in the Sony Reader Library software rather than in ADE, so perhaps it is relying on the location settings for my Reader (set to US). (They do show me prices in US $)
I've bought books from the Sony store using gift cards only. First time they asked for a credit card - and had some problems regarding the billing address for the card (which is not in the US, though it's a US credit card). Next time, they didn't bother with the credit card and just asked me for a "home" address - I gave the address of the friend who buys me the gift cards.
I have yet to try buying from the UK stores (Waterstones or WH Smiths) - though as they don't seem to be locked down to Sony, I suspect they'll sell to anyone.