Phones and phone cards are a bit special. Key reason might be that mobile networks only sign contracts for territory one country at a time currently. Because prices for phones and network time/data can differ greatly for the same product/service between countries, legally they must - and of course are very interested in - preventing arbitrage. So it's perfectly natural for networks to restrict the origin of methods of payment to the territory for which they have the relevant network licence.

Additionally as mentioned by other poster above there are quite serious fraud issues when accepting foreign credit cards, or cards of certain types, for some products and services (not just mobile but many other products).

As I mentioned in another thread here recently, it is extremely easy once in the UK to obtain top-ups at every supermarket, newspaper shops (eg WH Smith) most petrol stations and many other outlets (eg Boots pharmacies). Even SIMs are pretty easy to get hold of. Just say at the cash desk which network you want and how much £.